:root{
  --bg:#07090f;
  --panel:#11151f;
  --panel2:#151a25;
  --text:#f4f7fb;
  --muted:#9aa7ba;
  --line:rgba(255,255,255,.12);
  --accent:#8fc7ff;
  --accent2:#d7c3ff;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;
  font-family:Pretendard,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;
  background:
    radial-gradient(circle at 20% -10%, rgba(94,142,255,.22), transparent 34%),
    radial-gradient(circle at 90% 10%, rgba(186,137,255,.15), transparent 30%),
    var(--bg);
  color:var(--text);
}
a{text-decoration:none;color:inherit}
.wrap{width:min(1160px,92vw);margin:0 auto}
.nav{
  position:sticky;top:0;z-index:10;
  backdrop-filter:blur(18px);
  background:rgba(7,9,15,.72);
  border-bottom:1px solid var(--line);
}
.nav-inner{height:72px;display:flex;align-items:center;justify-content:space-between}
.brand{font-weight:900;letter-spacing:-.04em;font-size:22px}
.brand span{color:var(--accent)}
.menu{display:flex;gap:8px;align-items:center}
.menu a{
  padding:10px 14px;border-radius:999px;color:var(--muted);font-weight:700;font-size:14px
}
.menu a:hover,.menu a.active{background:rgba(255,255,255,.08);color:var(--text)}
.hero{padding:86px 0 72px}
.hero-grid{display:grid;grid-template-columns:1.05fr .95fr;gap:42px;align-items:center}
.eyebrow{color:var(--accent);font-weight:900;letter-spacing:.18em;font-size:13px}
h1{font-size:clamp(48px,7vw,86px);line-height:.98;letter-spacing:-.075em;margin:18px 0 24px}
.lead{font-size:19px;line-height:1.8;color:#c6d1e3;max-width:620px}
.cta-row{display:flex;gap:12px;margin-top:34px;flex-wrap:wrap}
.btn{
  display:inline-flex;align-items:center;justify-content:center;
  min-height:48px;padding:0 20px;border-radius:999px;
  font-weight:900;border:1px solid var(--line);background:rgba(255,255,255,.06)
}
.btn.primary{background:#f4f7fb;color:#07101d}
.btn:hover{transform:translateY(-2px)}
.hero-card{
  border:1px solid var(--line);
  border-radius:34px;
  background:linear-gradient(180deg,rgba(255,255,255,.08),rgba(255,255,255,.035));
  padding:28px;
  box-shadow:0 30px 90px rgba(0,0,0,.34);
}
.hero-visual{
  min-height:430px;border-radius:26px;background:
  linear-gradient(135deg,rgba(143,199,255,.18),rgba(215,195,255,.08)),
  #0c111a;
  position:relative;overflow:hidden;border:1px solid rgba(255,255,255,.08);
  display:flex;align-items:flex-end;
}
.hero-visual::before{
  content:"";position:absolute;inset:12%;
  background:radial-gradient(circle,rgba(143,199,255,.55),transparent 34%);
  filter:blur(18px);
}
.hero-visual-content{position:relative;z-index:1;padding:28px;width:100%}
.visual-title{font-size:38px;font-weight:950;letter-spacing:-.06em;margin:0 0 10px}
.visual-sub{color:#c8d1e1;margin:0;font-size:16px;line-height:1.7}
.stats{display:grid;grid-template-columns:repeat(3,1fr);gap:14px;margin-top:28px}
.stat{
  border:1px solid var(--line);border-radius:22px;background:rgba(255,255,255,.05);
  padding:20px
}
.stat b{font-size:28px;display:block}
.stat span{color:var(--muted);font-weight:700;font-size:13px}
.section{padding:46px 0}
.section-head{display:flex;justify-content:space-between;align-items:end;gap:22px;margin-bottom:22px}
.section h2{font-size:42px;line-height:1.05;letter-spacing:-.06em;margin:0}
.section p.sub{color:var(--muted);line-height:1.7;margin:10px 0 0}
.grid-3{display:grid;grid-template-columns:repeat(3,1fr);gap:18px}
.card{
  border:1px solid var(--line);border-radius:28px;background:rgba(255,255,255,.055);
  padding:24px;min-height:170px;transition:.18s;
}
.card:hover{transform:translateY(-5px);background:rgba(255,255,255,.08)}
.card small{display:block;color:var(--accent);font-weight:900;margin-bottom:18px}
.card h3{font-size:24px;letter-spacing:-.04em;margin:0 0 10px}
.card p{color:#b8c2d3;line-height:1.65;margin:0}
.lessons-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:18px}
.lesson-card{
  position:relative;overflow:hidden;
  border:1px solid var(--line);border-radius:30px;
  background:linear-gradient(180deg,rgba(255,255,255,.08),rgba(255,255,255,.035));
  padding:24px;min-height:220px;display:flex;flex-direction:column;justify-content:space-between;
  transition:.18s;
}
.lesson-card:hover{transform:translateY(-6px);border-color:rgba(143,199,255,.5)}
.lesson-no{font-size:13px;color:var(--accent);font-weight:950;letter-spacing:.16em}
.lesson-card h3{font-size:28px;line-height:1.15;letter-spacing:-.06em;margin:14px 0 0}
.lesson-meta{color:var(--muted);font-weight:700}
.page-hero{padding:70px 0 38px}
.page-title{font-size:clamp(42px,6vw,72px);letter-spacing:-.07em;line-height:1;margin:12px 0}
.two-col{display:grid;grid-template-columns:1.2fr .8fr;gap:22px;align-items:start}
.video-shell{
  border:1px solid var(--line);border-radius:32px;overflow:hidden;background:#05070b;
  box-shadow:0 24px 80px rgba(0,0,0,.32)
}
.video-box{position:relative;width:100%;padding-top:56.25%;background:#05070b}
.video-box iframe{position:absolute;inset:0;width:100%;height:100%}
.info-panel{
  border:1px solid var(--line);border-radius:32px;background:rgba(255,255,255,.055);
  padding:26px;
}
.info-panel h2{font-size:28px;margin:0 0 14px;letter-spacing:-.05em}
.info-panel p{color:#bcc7d7;line-height:1.75;margin:0}
.material-list{display:grid;gap:12px;margin-top:16px}
.material{
  display:flex;justify-content:space-between;align-items:center;gap:16px;
  padding:18px;border:1px solid var(--line);border-radius:20px;background:rgba(255,255,255,.045)
}
.material strong{font-size:16px}
.material span{color:var(--muted);font-size:13px;font-weight:700}
.material .tag{color:#07101d;background:#f4f7fb;border-radius:999px;padding:8px 12px;font-weight:900}
.material:hover{background:rgba(255,255,255,.08)}
.table{
  border:1px solid var(--line);border-radius:28px;overflow:hidden;background:rgba(255,255,255,.045)
}
.row{display:grid;grid-template-columns:90px 1fr 180px 180px;border-bottom:1px solid var(--line);align-items:center}
.row:last-child{border-bottom:0}
.cell{padding:18px;color:#c4cedd}
.cell.title{color:var(--text);font-weight:900}
.cell a{font-weight:900;color:var(--accent)}
.footer{border-top:1px solid var(--line);margin-top:70px;padding:32px 0;color:var(--muted)}
@media(max-width:900px){
  .hero-grid,.two-col{grid-template-columns:1fr}
  .grid-3,.lessons-grid{grid-template-columns:1fr}
  .section-head{display:block}
  .row{grid-template-columns:1fr}
  .cell{padding:12px 18px}
  .menu{gap:2px}
  .menu a{padding:9px 10px}
}

/* v7 problem-level lesson layout */
.lesson-layout{display:grid;grid-template-columns:minmax(0,1.25fr) 360px;gap:22px;align-items:start}
.side-stack{display:grid;gap:16px}
.problem-grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(160px,1fr));gap:14px}
.problem-card{border:1px solid var(--line);border-radius:24px;background:rgba(255,255,255,.055);padding:22px;min-height:150px;display:flex;flex-direction:column;justify-content:space-between;transition:.18s}
.problem-card:hover{transform:translateY(-4px);background:rgba(255,255,255,.085);border-color:rgba(143,199,255,.45)}
.problem-card b{font-size:22px;letter-spacing:-.04em}
.problem-number{font-size:34px;font-weight:900;color:#fff;letter-spacing:-.05em}
.problem-card span{color:var(--muted);font-weight:800}
.lesson-kicker{display:flex;gap:10px;flex-wrap:wrap;margin-top:16px}
.pill{border:1px solid var(--line);background:rgba(255,255,255,.055);color:#c6d1e3;border-radius:999px;padding:8px 12px;font-size:13px;font-weight:800}
@media(max-width:1050px){.lesson-layout{grid-template-columns:1fr}.problem-grid{grid-template-columns:repeat(auto-fit,minmax(150px,1fr))}}
@media(max-width:620px){.problem-grid{grid-template-columns:1fr}}


/* v8 problem number navigation */
.problem-nav{display:grid;grid-template-columns:repeat(auto-fit,minmax(72px,1fr));gap:10px}
.problem-nav a{border:1px solid var(--line);border-radius:16px;background:rgba(255,255,255,.055);padding:12px 10px;text-align:center;font-weight:900;color:#fff;transition:.18s}
.problem-nav a:hover{transform:translateY(-2px);background:rgba(255,255,255,.09);border-color:rgba(143,199,255,.45)}
.problem-nav a.active{background:rgba(143,199,255,.16);border-color:rgba(143,199,255,.55)}


/* v9 vertical problem list */
.problem-list-vertical{display:grid;grid-template-columns:1fr;gap:12px;max-width:860px}
.problem-row{display:grid;grid-template-columns:110px 1fr 42px;align-items:center;gap:18px;border:1px solid var(--line);border-radius:22px;background:rgba(255,255,255,.052);padding:18px 20px;transition:.18s}
.problem-row:hover{transform:translateY(-3px);background:rgba(255,255,255,.085);border-color:rgba(143,199,255,.45)}
.problem-row-no{color:var(--accent);font-weight:950;letter-spacing:.08em;font-size:13px}
.problem-row-main{display:flex;flex-direction:column;gap:4px}
.problem-row-main strong{font-size:22px;letter-spacing:-.04em}
.problem-row-main span{color:var(--muted);font-weight:750;font-size:14px}
.problem-row-arrow{justify-self:end;font-size:24px;font-weight:950;color:#fff}
.problem-nav.vertical{display:grid;grid-template-columns:1fr;gap:10px}
.problem-nav.vertical a{text-align:left;padding:14px 16px;border-radius:18px}
.empty-video{min-height:420px;display:flex;flex-direction:column;align-items:center;justify-content:center;gap:12px;text-align:center;background:#05070b;border-radius:32px;padding:32px;color:var(--text)}
.empty-video span{color:var(--muted);line-height:1.6}
@media(max-width:620px){.problem-row{grid-template-columns:1fr 32px;gap:10px}.problem-row-no{grid-column:1 / -1}.problem-row-main strong{font-size:20px}}

.empty-state{border:1px solid var(--line);border-radius:24px;background:rgba(255,255,255,.052);padding:28px;display:flex;flex-direction:column;gap:8px;color:#fff}
.empty-state strong{font-size:20px;letter-spacing:-.03em}
.empty-state span{color:var(--muted);font-weight:750}