:root{
  --paper:#f7f3e9;
  --paper-2:#fbf8f1;
  --ink:#1f2430;
  --ink-soft:#5b6273;
  --ink-faint:#8d93a3;
  --pencil:#2f3a56;
  --red:#c2452f;
  --marker:rgba(255,214,102,.55);
  --line:#e2dccc;
  --accent:#2f5fd0;
  --ease:cubic-bezier(.22,1,.36,1);
  --ease-spring:cubic-bezier(.34,1.45,.64,1);
}

*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;
  background:
    radial-gradient(1200px 700px at 12% -10%, #fffdf7 0%, transparent 60%),
    radial-gradient(900px 600px at 105% 20%, #f3eee1 0%, transparent 55%),
    var(--paper);
  color:var(--ink);
  font-family:"Literata",Georgia,"Times New Roman",serif;
  font-size:17px;
  line-height:1.65;
  padding:0 0 80px;
}

/* лёгкая шершавость бумаги */
body::before{
  content:"";position:fixed;inset:0;pointer-events:none;z-index:0;opacity:.35;
  background-image:radial-gradient(circle at 20% 30%, rgba(0,0,0,.025) 0 1px, transparent 1px),
                   radial-gradient(circle at 70% 60%, rgba(0,0,0,.02) 0 1px, transparent 1px);
  background-size:7px 7px, 11px 11px;
}

.nav{
  position:sticky;top:0;z-index:5;display:flex;gap:14px;align-items:center;flex-wrap:wrap;
  padding:10px 20px;background:rgba(247,243,233,.92);backdrop-filter:blur(6px);
  border-bottom:1px solid var(--line);font-size:14px;
}
.nav-title{font-family:"Caveat",cursive;font-size:22px;margin-right:6px;color:var(--pencil)}
.nav a{color:var(--ink-soft);text-decoration:none;padding:3px 2px;border-bottom:1.5px solid transparent;transition:color .2s var(--ease),border-color .2s var(--ease)}
.nav a:hover,.nav a.on{color:var(--red);border-color:var(--red)}

.sheet{
  position:relative;z-index:1;
  max-width:760px;margin:26px auto;padding:34px 34px 34px 64px;
  background:var(--paper-2);
  border:1px solid var(--line);
  border-radius:4px;
  scroll-margin-top:86px;
  box-shadow:0 1px 0 rgba(0,0,0,.03), 0 18px 40px -32px rgba(31,36,48,.5);
}
/* красное поле тетради, без клеток */
.sheet::before{content:"";position:absolute;left:44px;top:0;bottom:0;width:1.5px;background:rgba(194,69,47,.35)}

.sheet.cover{padding-top:44px}
.date{margin:0;color:var(--ink-faint);font-size:14px;letter-spacing:.02em}
h1{font-size:44px;line-height:1.1;margin:10px 0 6px;font-weight:600;letter-spacing:-.01em}
h2{font-size:28px;line-height:1.2;margin:4px 0 14px;font-weight:600}
.tag{margin:0;font-family:"Caveat",cursive;font-size:21px;color:var(--red);transform:rotate(-1.5deg);transform-origin:left}
.hand{font-family:"Caveat",cursive;font-size:22px;color:var(--pencil)}
.cover-note{margin:2px 0 18px;transform:rotate(-.7deg)}
.cover-hint{color:var(--ink-soft);font-size:15px;margin:18px 0 0}

.toc{list-style:none;margin:0;padding:0;border-top:1px solid var(--line)}
.toc li{border-bottom:1px solid var(--line)}
.toc a{display:flex;gap:12px;align-items:baseline;padding:11px 4px;color:var(--ink);text-decoration:none;transition:transform .25s var(--ease),color .2s var(--ease)}
.toc a span{font-family:"Caveat",cursive;font-size:22px;color:var(--red);min-width:18px}
.toc a:hover{transform:translateX(6px);color:var(--red)}

.rule{background:rgba(47,95,208,.06);border-left:3px solid var(--accent);padding:10px 14px;margin:0 0 18px;border-radius:0 8px 8px 0;font-size:16px}
.rule b{font-weight:600}

.plan{display:flex;flex-wrap:wrap;gap:8px;margin:0 0 18px}
.plan span{font-size:13.5px;color:var(--ink-soft);background:rgba(31,36,48,.05);border-radius:99px;padding:4px 12px}

.work{margin:0 0 22px}
.work-title{font-family:"Caveat",cursive;font-size:23px;color:var(--pencil);margin:0 0 10px}
.step{display:flex;gap:14px;padding:10px 0;border-bottom:1px dashed var(--line)}
.step:last-of-type{border-bottom:0}
.num{flex:none;width:26px;height:26px;border-radius:50%;border:1.5px solid var(--red);color:var(--red);
     display:flex;align-items:center;justify-content:center;font-size:14px;font-family:"Caveat",cursive}
.body{flex:1;min-width:0}
.math{font-family:"Literata",Georgia,serif;font-size:17px;line-height:1.85;white-space:pre-wrap;letter-spacing:.01em}
mark{background:linear-gradient(transparent 55%, var(--marker) 55%);padding:0 2px;color:inherit}
.why{margin:6px 0 0;font-size:14px;line-height:1.55;color:var(--ink-soft);font-style:italic}
.result{margin:12px 0 0;font-size:17px;font-weight:600}
.note{margin:14px 0 0;transform:rotate(-.4deg)}

.pair{display:grid;grid-template-columns:1fr 1fr;gap:10px;margin:14px 0 0}
.pair div{border-radius:10px;padding:10px 12px;font-size:15px;line-height:1.5}
.pair span{display:block;font-family:"Caveat",cursive;font-size:18px;margin-bottom:2px}
.bad{background:rgba(194,69,47,.09)}
.bad span{color:var(--red)}
.good{background:rgba(47,138,92,.1)}
.good span{color:#2f8a5c}

.try{margin:20px 0 0;padding:14px 16px;background:rgba(31,36,48,.035);border-radius:12px}
.try-q{margin:0 0 10px;font-size:16px}
.reveal{font:inherit;font-size:14px;background:transparent;border:1.5px solid var(--pencil);color:var(--pencil);
        border-radius:99px;padding:5px 14px;cursor:pointer;transition:background .2s var(--ease),color .2s var(--ease),transform .2s var(--ease-spring)}
.reveal:hover{background:var(--pencil);color:var(--paper-2);transform:translateY(-1px)}
.hidden{display:none;margin-top:12px}
.hidden.open{display:block;animation:unfold .35s var(--ease)}
@keyframes unfold{from{opacity:0;transform:translateY(-4px)}to{opacity:1;transform:none}}

/* числовая прямая */
.axis{position:relative;height:64px;margin:6px 0 2px}
.axis::before{content:"";position:absolute;left:0;right:0;top:34px;height:1.5px;background:var(--pencil)}
.axis::after{content:"";position:absolute;right:0;top:29px;border:6px solid transparent;border-left-color:var(--pencil)}
.pt{position:absolute;top:24px;transform:translateX(-50%);font-size:14px;color:var(--ink);padding-top:18px;text-align:center}
.pt::before{content:"";position:absolute;left:50%;top:4px;width:11px;height:11px;border-radius:50%;transform:translateX(-50%);border:1.5px solid var(--pencil)}
.pt.filled::before{background:var(--pencil)}
.pt.hollow::before{background:var(--paper-2)}
.sg{position:absolute;top:0;font-style:normal;font-size:20px;color:var(--red);font-family:"Caveat",cursive}

.check{margin:0;padding-left:22px}
.check li{margin:0 0 9px;font-size:16px}
.sign{display:block;margin-top:18px;transform:rotate(-1deg);color:var(--red)}

.foot{max-width:760px;margin:0 auto;padding:0 20px;display:flex;gap:14px;align-items:center;flex-wrap:wrap;
      color:var(--ink-faint);font-size:13.5px}
#print{font:inherit;font-size:14px;background:var(--pencil);color:var(--paper-2);border:0;border-radius:99px;
       padding:8px 16px;cursor:pointer;transition:transform .2s var(--ease-spring),opacity .2s var(--ease)}
#print:hover{transform:translateY(-1px);opacity:.92}

/* появление при прокрутке */
.sheet{opacity:0;transform:translateY(14px);transition:opacity .6s var(--ease),transform .6s var(--ease)}
.sheet.seen{opacity:1;transform:none}

@media (max-width:720px){
  body{font-size:16px}
  .sheet{margin:16px 12px;padding:24px 18px 26px 44px;border-radius:3px}
  .sheet::before{left:28px}
  h1{font-size:34px}
  h2{font-size:24px}
  .pair{grid-template-columns:1fr}
  .nav{gap:14px;font-size:13px;padding:9px 14px;flex-wrap:nowrap;overflow-x:auto;scrollbar-width:none}
  .nav::-webkit-scrollbar{display:none}
  .nav a{white-space:nowrap}
  .sheet{scroll-margin-top:60px}
  .math{font-size:16px}
}

@media (prefers-reduced-motion:reduce){
  *{animation:none!important;transition:none!important}
  .sheet{opacity:1;transform:none}
  html{scroll-behavior:auto}
}

@media print{
  body{background:#fff;padding:0}
  body::before,.nav,.foot,.reveal{display:none}
  .sheet{box-shadow:none;border:0;opacity:1;transform:none;margin:0;padding:12mm 10mm 12mm 18mm;page-break-after:always}
  .hidden{display:block!important}
}