:root {
  --bg: #0c1114;
  --surface: #151c20;
  --surface-2: #1c252a;
  --surface-3: #10171b;
  --line: #34434b;
  --line-strong: #50616a;
  --text: #f4f7f8;
  --muted: #aab7be;
  --green: #61e3a5;
  --blue: #6bbcff;
  --yellow: #ffd166;
  --red: #ff7b7b;
  --purple: #b8a1ff;
  --ink: #0c1114;
  --code: #090e11;
  --content: 1160px;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: Inter, system-ui, -apple-system, "Segoe UI", Arial, sans-serif;
  line-height: 1.65;
}

button, input, select, textarea { font: inherit; }
a { color: var(--blue); }
img { max-width: 100%; }
code, pre, .mono { font-family: Consolas, "Courier New", monospace; }

a:focus-visible, button:focus-visible, input:focus-visible, select:focus-visible,
summary:focus-visible {
  outline: 3px solid rgba(107, 188, 255, .4);
  outline-offset: 3px;
}

.site-header {
  position: sticky;
  z-index: 20;
  top: 0;
  border-bottom: 1px solid var(--line);
  background: rgba(12, 17, 20, .96);
  backdrop-filter: blur(12px);
}

.topbar {
  display: flex;
  width: min(var(--content), 100%);
  min-height: 60px;
  margin: 0 auto;
  padding: 0 20px;
  align-items: center;
  justify-content: space-between;
  gap: 18px;
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  color: var(--text);
  font-weight: 900;
  text-decoration: none;
}

.brand-mark {
  display: grid;
  width: 32px;
  height: 32px;
  place-items: center;
  border: 1px solid #4a725f;
  border-radius: 6px;
  background: #143426;
  color: var(--green);
  font: 900 .9rem/1 Consolas, monospace;
}

.topnav { display: flex; align-items: center; gap: 7px; }
.topnav a {
  padding: 7px 9px;
  border-radius: 5px;
  color: var(--muted);
  font-size: .86rem;
  font-weight: 800;
  text-decoration: none;
}
.topnav a:hover { background: var(--surface-2); color: var(--text); }

.layout {
  display: grid;
  width: min(var(--content), 100%);
  margin: 0 auto;
  padding: 26px 20px 64px;
  grid-template-columns: 230px minmax(0, 1fr);
  gap: 30px;
}

.sidebar {
  position: sticky;
  top: 84px;
  max-height: calc(100vh - 108px);
  overflow-y: auto;
  align-self: start;
  padding-right: 10px;
}

.sidebar-title {
  margin: 0 0 9px;
  color: var(--muted);
  font-size: .72rem;
  font-weight: 900;
  text-transform: uppercase;
}

.course-menu { display: grid; gap: 3px; }
.course-menu a {
  display: grid;
  grid-template-columns: 28px minmax(0, 1fr);
  gap: 7px;
  padding: 7px 8px;
  border-left: 2px solid transparent;
  color: var(--muted);
  font-size: .81rem;
  line-height: 1.3;
  text-decoration: none;
}
.course-menu a:hover { color: var(--text); background: var(--surface); }
.course-menu a.active { border-left-color: var(--green); background: #14231d; color: var(--text); }
.course-menu .menu-number { color: var(--green); font-family: Consolas, monospace; }

.main { min-width: 0; }
.lesson-header { padding: 10px 0 28px; }
.eyebrow {
  margin: 0 0 7px;
  color: var(--green);
  font-size: .77rem;
  font-weight: 900;
  text-transform: uppercase;
}

h1, h2, h3, h4, p { margin-top: 0; }
h1 {
  max-width: 900px;
  margin-bottom: 12px;
  font-size: clamp(2rem, 4vw, 3.25rem);
  line-height: 1.07;
  letter-spacing: 0;
}
h2 { margin-bottom: 12px; font-size: 1.48rem; line-height: 1.2; letter-spacing: 0; }
h3 { margin-bottom: 8px; font-size: 1.06rem; letter-spacing: 0; }
h4 { margin-bottom: 6px; font-size: .95rem; letter-spacing: 0; }
p { color: var(--muted); }
.lead { max-width: 850px; margin-bottom: 0; color: #d4dce0; font-size: 1.05rem; }

.lesson-meta { display: flex; flex-wrap: wrap; gap: 8px; margin-top: 18px; }
.badge {
  display: inline-flex;
  min-height: 30px;
  padding: 4px 9px;
  align-items: center;
  border: 1px solid var(--line);
  border-radius: 5px;
  color: var(--muted);
  font-size: .8rem;
  font-weight: 800;
}
.badge.green { border-color: #416b57; color: var(--green); }
.badge.yellow { border-color: #6b5d36; color: var(--yellow); }

.progress-track { height: 4px; margin-top: 22px; overflow: hidden; background: var(--surface-2); }
.progress-track span { display: block; height: 100%; background: var(--green); }

.band { min-width: 0; padding: 29px 0; border-top: 1px solid var(--line); }
.band > :last-child { margin-bottom: 0; }
.two-col { display: grid; grid-template-columns: minmax(0, 1fr) minmax(300px, .85fr); gap: 28px; align-items: start; }
.three-col { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 12px; }

.concept-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 11px; }
.concept-card, .problem-card {
  min-width: 0;
  padding: 15px;
  border: 1px solid var(--line);
  border-radius: 7px;
  background: var(--surface);
}
.concept-card h3, .problem-card h3 { color: var(--text); }
.concept-card p, .problem-card p { margin-bottom: 0; font-size: .92rem; }
.concept-card.future { border-style: dashed; }
.future-label { display: inline-block; margin-bottom: 8px; color: var(--yellow); font-size: .72rem; font-weight: 900; text-transform: uppercase; }

.flow {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 18px 0;
  align-items: center;
}
.flow-step {
  padding: 9px 11px;
  border: 1px solid var(--line);
  border-radius: 5px;
  background: var(--surface);
  color: var(--muted);
  font-size: .82rem;
  font-weight: 800;
}
.flow-step.active { border-color: var(--yellow); background: #332c16; color: var(--yellow); }
.flow-arrow { color: var(--green); font-weight: 900; }

.note, .warning, .success {
  margin: 16px 0;
  padding: 14px 16px;
  border-radius: 6px;
  color: var(--muted);
}
.note { border-left: 3px solid var(--blue); background: #142633; }
.warning { border-left: 3px solid var(--red); background: #2b1a1e; }
.success { border-left: 3px solid var(--green); background: #14271e; }
.note strong { color: var(--blue); }
.warning strong { color: var(--red); }
.success strong { color: var(--green); }

.clean-list, .steps, .sources, .error-list { display: grid; gap: 9px; margin: 0; padding: 0; list-style: none; }
.clean-list li, .steps li, .error-list li { padding: 10px 0 10px 15px; border-left: 3px solid var(--blue); color: var(--muted); }
.steps { counter-reset: steps; }
.steps li { counter-increment: steps; }
.steps li::before { content: counter(steps) ". "; color: var(--yellow); font-weight: 900; }
.clean-list strong, .steps strong, .error-list strong { color: var(--text); }
.error-list li { border-left-color: var(--red); }

.code-shell { overflow: hidden; border: 1px solid var(--line); border-radius: 7px; background: var(--code); }
.code-head { display: flex; padding: 9px 13px; align-items: center; justify-content: space-between; gap: 12px; border-bottom: 1px solid var(--line); color: var(--muted); font-size: .78rem; }
pre { margin: 0; padding: 16px; overflow-x: auto; color: #dce5e9; font: .88rem/1.65 Consolas, "Courier New", monospace; tab-size: 4; }
.copy-btn { padding: 5px 8px; border: 1px solid var(--line); border-radius: 4px; background: var(--surface-2); color: var(--text); cursor: pointer; font-size: .75rem; font-weight: 800; }
.copy-btn:hover { border-color: var(--blue); }

.io-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
.io-box { min-width: 0; border: 1px solid var(--line); border-radius: 6px; background: var(--surface-3); }
.io-box h3 { margin: 0; padding: 8px 12px; border-bottom: 1px solid var(--line); color: var(--muted); font-size: .76rem; text-transform: uppercase; }
.io-box pre { min-height: 80px; }

.trace { display: grid; gap: 8px; }
.trace-row { display: grid; grid-template-columns: 36px minmax(0, 1fr); gap: 10px; align-items: start; }
.trace-number { display: grid; width: 32px; height: 32px; place-items: center; border: 1px solid #55636b; border-radius: 50%; color: var(--yellow); font: 800 .82rem/1 Consolas, monospace; }
.trace-copy { padding: 4px 0; color: var(--muted); }
.trace-copy code { color: var(--green); }

.table-wrap { max-width: 100%; overflow-x: auto; border: 1px solid var(--line); border-radius: 7px; }
table { width: 100%; border-collapse: collapse; font-size: .9rem; }
th, td { padding: 10px 12px; border-bottom: 1px solid var(--line); text-align: left; vertical-align: top; }
th { background: var(--surface-2); color: var(--text); font-size: .78rem; text-transform: uppercase; }
td { color: var(--muted); }
tr:last-child td { border-bottom: 0; }

.lab { overflow: hidden; border: 1px solid var(--line); border-radius: 8px; background: var(--surface); }
.lab-header { display: flex; padding: 14px 16px; align-items: center; justify-content: space-between; gap: 15px; border-bottom: 1px solid var(--line); }
.lab-header h2 { margin: 0; font-size: 1.08rem; }
.lab-body { min-height: 260px; padding: 18px 16px; }
.lab-controls { display: flex; flex-wrap: wrap; gap: 10px; padding: 13px 16px; border-top: 1px solid var(--line); background: var(--surface-2); }
.field { display: grid; flex: 1 1 180px; gap: 4px; min-width: 0; }
.field label { color: var(--muted); font-size: .72rem; font-weight: 900; text-transform: uppercase; }
.field input, .field select, .field textarea {
  width: 100%; min-height: 42px; padding: 8px 10px; border: 1px solid var(--line-strong); border-radius: 5px; background: #0d1418; color: var(--text);
}
.field textarea { min-height: 116px; resize: vertical; font: .86rem/1.5 Consolas, "Courier New", monospace; }
.input-mode-field { flex: 0 1 260px; }
.input-data-field { flex-basis: 320px; }
.btn { display: inline-flex; min-height: 42px; padding: 8px 13px; align-items: center; justify-content: center; gap: 7px; border: 1px solid var(--line-strong); border-radius: 6px; background: #27333a; color: var(--text); font-weight: 850; text-decoration: none; cursor: pointer; }
.btn:hover { border-color: var(--blue); }
.btn.primary { border-color: var(--green); background: var(--green); color: var(--ink); }
.btn.secondary { border-color: #685a34; background: #332d1c; color: var(--yellow); }
.btn:disabled { opacity: .4; cursor: default; }
.btn-row { display: flex; flex-wrap: wrap; gap: 9px; }

.tokens { display: flex; min-height: 72px; flex-wrap: wrap; gap: 7px; align-content: flex-start; }
.token { display: grid; min-width: 47px; height: 50px; padding: 0 8px; place-items: center; border: 1px solid var(--line-strong); border-radius: 5px; background: var(--surface-2); color: var(--text); font: 850 .95rem/1 Consolas, monospace; }
.token.active { border-color: var(--yellow); background: #3a3117; color: var(--yellow); }
.token.done { border-color: #416a57; background: #153426; color: var(--green); }
.token.bad { border-color: #704247; background: #351c20; color: var(--red); }
.token-index { display: block; color: var(--muted); font-size: .65rem; }
.lab-message { min-height: 46px; margin-top: 14px; padding: 11px 13px; border-left: 3px solid var(--green); background: var(--surface-3); color: var(--muted); }
.lab-message strong { color: var(--text); }

.input-simulation { display: grid; grid-template-columns: minmax(0, 1.35fr) minmax(210px, .65fr); gap: 18px; align-items: start; }
.input-lines { display: grid; gap: 7px; }
.input-line {
  display: grid;
  grid-template-columns: 32px minmax(0, 1fr) auto;
  min-height: 44px;
  align-items: center;
  gap: 10px;
  padding: 7px 10px;
  border: 1px solid var(--line);
  border-radius: 5px;
  background: var(--surface-3);
  color: var(--muted);
}
.input-line.current { border-color: var(--yellow); background: #332c16; }
.input-line.consumed { border-color: #416a57; background: #153426; }
.input-line.eof { border-style: dashed; }
.input-line-number { display: grid; width: 27px; height: 27px; place-items: center; border-radius: 50%; background: var(--surface-2); color: var(--muted); font: 800 .72rem/1 Consolas, monospace; }
.input-line code { min-width: 0; overflow-wrap: anywhere; color: var(--text); }
.input-status { color: var(--muted); font-size: .7rem; font-weight: 850; text-transform: uppercase; }
.input-output { display: grid; min-height: 72px; gap: 7px; padding: 11px; border: 1px solid var(--line); border-radius: 5px; background: var(--surface-3); align-content: start; }
.input-output code { color: var(--green); }
.input-output span { color: var(--muted); }

.stats-grid {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 8px;
  margin-top: 16px;
}

.stats-grid.compact { grid-template-columns: repeat(2, minmax(0, 1fr)); }

.stat-card {
  display: flex;
  min-width: 0;
  min-height: 92px;
  flex-direction: column;
  justify-content: center;
  padding: 12px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--surface-2);
}

.stat-card span,
.stat-card small { color: var(--muted); font-size: .72rem; }
.stat-card strong { margin: 4px 0; color: var(--green); font-size: 1.2rem; overflow-wrap: anywhere; }

.frequency-list {
  display: grid;
  gap: 7px;
  margin-top: 16px;
}

.frequency-row {
  display: grid;
  grid-template-columns: 62px minmax(90px, 1fr) 72px;
  gap: 10px;
  align-items: center;
  color: var(--muted);
  font-size: .78rem;
}

.frequency-row strong { color: var(--text); font-family: Consolas, monospace; }
.frequency-track { height: 10px; overflow: hidden; border-radius: 4px; background: var(--surface-3); }
.frequency-track span { display: block; height: 100%; background: var(--blue); }

.scoreboard { display: grid; gap: 7px; }
.score-row { display: grid; grid-template-columns: 32px minmax(110px, 1fr) 75px 88px; gap: 8px; padding: 9px 10px; align-items: center; border: 1px solid var(--line); border-radius: 5px; color: var(--muted); }
.score-row.current { border-color: var(--yellow); background: #292418; }
.score-row strong { color: var(--text); }

.verdict-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(145px, 1fr)); gap: 8px; }
.verdict { padding: 12px; border: 1px solid var(--line); border-radius: 6px; background: var(--surface-3); cursor: pointer; }
.verdict:hover { border-color: var(--blue); }
.verdict strong { display: block; color: var(--text); }
.verdict span { color: var(--muted); font-size: .8rem; }

.challenge { padding: 18px; border: 1px solid #695b35; border-radius: 7px; background: #252118; }
.challenge h2 { color: var(--yellow); }
details.solution { margin-top: 14px; border-top: 1px solid #695b35; }
details.solution summary { padding: 12px 0 0; color: var(--yellow); font-weight: 850; cursor: pointer; }
details.solution > div { padding-top: 12px; }

.sources li { color: var(--muted); }
.sources a { font-weight: 750; }
.source-note { color: var(--muted); font-size: .83rem; }

.lesson-nav { display: grid; grid-template-columns: 1fr 1fr; gap: 11px; padding-top: 28px; border-top: 1px solid var(--line); }
.lesson-nav a { min-width: 0; padding: 12px 14px; border: 1px solid var(--line); border-radius: 6px; color: var(--text); font-weight: 850; text-decoration: none; }
.lesson-nav a:last-child { text-align: right; }
.lesson-nav a:hover { border-color: var(--blue); }

.problem-title-row { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; }
.problem-number { color: var(--green); font: 900 .95rem/1 Consolas, monospace; }
.problem-actions { display: flex; flex-wrap: wrap; gap: 9px; margin-top: 17px; }
.problem-list { display: grid; gap: 10px; }
.problem-card { display: grid; grid-template-columns: 82px minmax(0, 1fr) auto; gap: 14px; align-items: center; }
.problem-id { color: var(--green); font: 900 1rem/1 Consolas, monospace; }
.problem-card a { color: var(--text); text-decoration: none; }
.problem-card a:hover { color: var(--blue); }
.difficulty { color: var(--yellow); font-size: .78rem; font-weight: 850; white-space: nowrap; }

.site-footer { border-top: 1px solid var(--line); }
.site-footer-inner { display: flex; width: min(var(--content), 100%); margin: 0 auto; padding: 22px 20px; justify-content: space-between; gap: 15px; color: var(--muted); font-size: .82rem; }

@media (max-width: 900px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar { position: static; max-height: none; padding: 0 0 18px; border-bottom: 1px solid var(--line); }
  .course-menu { display: flex; overflow-x: auto; padding-bottom: 6px; }
  .course-menu a { min-width: 180px; border-left: 0; border-bottom: 2px solid transparent; }
  .course-menu a.active { border-left: 0; border-bottom-color: var(--green); }
}

@media (max-width: 680px) {
  .topbar { min-height: 54px; padding: 0 14px; }
  .brand span:last-child { display: none; }
  .topnav a { padding: 6px; font-size: .78rem; }
  .layout { padding: 18px 14px 46px; gap: 22px; }
  h1 { font-size: 2.08rem; }
  .two-col, .three-col, .concept-grid, .io-grid, .input-simulation { grid-template-columns: 1fr; }
  .problem-card { grid-template-columns: 68px minmax(0, 1fr); }
  .problem-card .difficulty { grid-column: 2; }
  .score-row { grid-template-columns: 26px minmax(90px, 1fr) 54px 62px; font-size: .8rem; }
  .lab-body { min-height: 0; padding: 14px; }
  .field { flex-basis: 100%; }
  .input-line { grid-template-columns: 30px minmax(0, 1fr); }
  .input-status { grid-column: 2; }
  .lab-controls .btn { flex: 1 1 120px; }
  .stats-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .lesson-nav { grid-template-columns: 1fr; }
  .lesson-nav a:last-child { text-align: left; }
  .site-footer-inner { flex-direction: column; }
}

@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }