@font-face{font-family:Unbounded;src:url(/fonts/unbounded-cyrillic-700-normal.woff2)format("woff2");font-weight:700;font-display:swap;unicode-range:U+301,U+400-45F,U+490-491,U+4B0-4B1,U+2116}@font-face{font-family:Unbounded;src:url(/fonts/unbounded-latin-700-normal.woff2)format("woff2");font-weight:700;font-display:swap;unicode-range:U+??,U+2013-2014,U+2018-201E,U+2026,U+20BD}:root{--sheet:#e6ebf1;--grid:#d5dde6;--paper:#fff;--ink:#1c2150;--ink-soft:#4a5180;--pen:#2b3ff0;--marker:#ffe14d;--check:#1e9e63;--g1:#ffb0d4;--g2:#ffc56b;--g3:#9be7c4;--g4:#a9c8ff;--g5:#ff8fc7;--g6:#ffb23f;--g7:#ffe14d;--g8:#5be0a8;--g9:#6fa8ff;--g10:#b38bff;--g11:#ff7a6b;--line:2px solid var(--ink);--cut:4px 4px 0 var(--ink);--cut-sm:3px 3px 0 var(--ink);--r:4px;--display:"Unbounded", system-ui, sans-serif;--text:system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif}@media (prefers-color-scheme:dark){:root{--sheet:#151936;--grid:#1d2246;--paper:#222852;--ink:#f1f3ff;--ink-soft:#b7bde3;--pen:#9aa8ff;--check:#5be0a8;--cut:4px 4px 0 #050714;--cut-sm:3px 3px 0 #050714}.btn.primary,.next-step,.btn.primary:hover,.next-step:hover{color:#151936}}*{box-sizing:border-box}html{-webkit-text-size-adjust:100%}body{color:var(--ink);font:17px/1.6 var(--text);background-color:var(--sheet);background-image:linear-gradient(var(--grid) 1px, transparent 1px), linear-gradient(90deg, var(--grid) 1px, transparent 1px);background-size:24px 24px;margin:0}a{color:var(--pen);text-underline-offset:3px;text-decoration:underline 2px}a:hover{text-decoration-color:var(--marker)}:focus-visible{outline:3px solid var(--pen);outline-offset:3px}h1,h2,.logo,.grade-tile span,.task-grid a,.task-grid span,.hero-count strong{font-family:var(--display);letter-spacing:-.01em;font-weight:700}h1{margin:.8rem 0 1.1rem;font-size:1.6rem;line-height:1.2}h2{margin:2rem 0 .8rem;font-size:1.15rem;line-height:1.3}h2 a{color:inherit;text-decoration:none}h3{margin:0 0 .3rem;font-size:1rem}.muted{color:var(--ink-soft);font-size:.92em}.wrap{max-width:800px;margin:0 auto;padding:0 16px}main.wrap{min-height:60vh;padding-bottom:48px}[data-g="1"]{--g:var(--g1)}[data-g="2"]{--g:var(--g2)}[data-g="3"]{--g:var(--g3)}[data-g="4"]{--g:var(--g4)}[data-g="5"]{--g:var(--g5)}[data-g="6"]{--g:var(--g6)}[data-g="7"]{--g:var(--g7)}[data-g="8"]{--g:var(--g8)}[data-g="9"]{--g:var(--g9)}[data-g="10"]{--g:var(--g10)}[data-g="11"]{--g:var(--g11)}.site-header{background:var(--paper);border-bottom:var(--line);z-index:10;position:sticky;top:0}.header-row{align-items:center;gap:14px;height:60px;display:flex}.logo{color:var(--ink);white-space:nowrap;align-items:center;gap:10px;font-size:1.05rem;text-decoration:none;display:flex}.logo:hover{text-decoration:none}.grade-nav{scrollbar-width:none;gap:6px;margin-left:auto;padding:4px 4px 6px 0;display:flex;overflow-x:auto}.grade-nav::-webkit-scrollbar{display:none}.grade-nav a{white-space:nowrap;color:#1c2150;background:var(--g,var(--paper));border:var(--line);border-radius:var(--r);padding:3px 9px;font-size:.85rem;font-weight:700;text-decoration:none}.grade-nav a:hover{box-shadow:var(--cut-sm)}.site-footer{border-top:var(--line);background:var(--paper);color:var(--ink-soft);font-size:.9rem}.footer-row{flex-direction:column;gap:8px;padding:22px 16px;display:flex}.footer-row p{margin:0}.footer-row nav{flex-wrap:wrap;gap:6px 18px;display:flex}.footer-row a{color:var(--ink)}.crumbs{color:var(--ink-soft);flex-wrap:wrap;gap:4px 6px;margin-top:16px;font-size:.85rem;display:flex}.crumbs a{color:var(--ink-soft);text-decoration-thickness:1px}.crumbs .sep{opacity:.6}.hero{background:var(--marker);color:#1c2150;border-radius:var(--r);box-shadow:var(--cut);border:2px solid #1c2150;margin-top:22px;padding:26px 22px 22px}.hero h1{max-width:12ch;margin:0 0 14px;font-size:clamp(1.9rem,7vw,3rem);line-height:1.05}.hero-lead{max-width:34em;margin:0 0 18px;font-size:1.1rem}.hero-count{border-radius:var(--r);background:#fff;border:2px solid #1c2150;align-items:baseline;gap:10px;margin:0;padding:6px 12px;font-size:.92rem;display:inline-flex}.hero-count strong{font-size:1.15rem}.grade-grid{grid-template-columns:repeat(auto-fill,minmax(84px,1fr));gap:12px;display:grid}.grade-tile{aspect-ratio:1;background:var(--g,var(--paper));color:#1c2150;border-radius:var(--r);box-shadow:var(--cut);border:2px solid #1c2150;flex-direction:column;justify-content:space-between;align-items:flex-start;padding:10px 12px;text-decoration:none;display:flex}.grade-tile span{font-size:2.2rem;line-height:1}.grade-tile small{font-size:.8rem;font-weight:600}.grade-tile:hover{text-decoration:none}.grade-tile:active,.book-card:active,.btn:active,.task-grid a:active{box-shadow:none;transform:translate(3px,3px)}.book-list{grid-template-columns:repeat(auto-fill,minmax(270px,1fr));gap:14px;display:grid}.book-card{background:var(--paper);color:var(--ink);border:var(--line);border-radius:var(--r);box-shadow:var(--cut);align-items:center;gap:14px;padding:12px;text-decoration:none;display:flex}.book-card:hover{text-decoration:none}.book-cover{background:var(--g,var(--marker));color:#1c2150;width:54px;height:72px;font-family:var(--display);border:2px solid #1c2150;border-radius:2px;flex:none;place-items:center;font-size:1.5rem;font-weight:700;display:grid;overflow:hidden}.book-cover img{object-fit:cover;width:100%;height:100%}.book-cover.big{width:96px;height:128px;box-shadow:var(--cut);font-size:2.4rem}.book-info{flex-direction:column;line-height:1.35;display:flex}.book-head{align-items:flex-start;gap:18px;display:flex}.book-meta{grid-template-columns:auto 1fr;gap:3px 14px;margin:0;font-size:.92rem;display:grid}.book-meta dt{color:var(--ink-soft)}.book-meta dd{margin:0}.lead{margin-top:1.1rem}.task-grid{grid-template-columns:repeat(auto-fill,minmax(56px,1fr));gap:8px;display:grid}.task-grid a,.task-grid span{text-align:center;font-variant-numeric:tabular-nums;background:var(--paper);color:var(--ink);border:var(--line);border-radius:var(--r);padding:9px 0;font-size:.9rem;text-decoration:none}.task-grid a:hover{box-shadow:var(--cut-sm);background:var(--marker);color:#1c2150}.task-grid span[aria-current]{background:var(--ink);color:var(--paper)}.task-grid.near{grid-template-columns:repeat(auto-fill,minmax(46px,1fr));gap:6px;margin:16px 0}.task-grid.near a,.task-grid.near span{padding:7px 0;font-size:.85rem}.part-title{background:var(--ink);color:var(--paper);border-radius:var(--r);margin-top:2.2rem;padding:6px 12px;display:inline-block}.task-section h3{margin:1.2rem 0 .6rem;font-size:1rem}.book-strip{background:var(--paper);color:var(--ink);border:var(--line);border-radius:var(--r);flex-direction:column;padding:10px 14px;line-height:1.4;text-decoration:none;display:flex}.book-strip:hover{box-shadow:var(--cut-sm)}.edition-tabs{flex-wrap:wrap;gap:8px;margin:14px 0;display:flex}.edition-tabs a{background:var(--paper);color:var(--ink);border:var(--line);border-radius:var(--r);padding:5px 12px;font-size:.9rem;font-weight:600;text-decoration:none}.solution{margin-top:16px}.solution+.solution{border-top:2px dashed var(--ink);padding-top:10px}.edition-title{margin-bottom:.4rem}.condition,.steps,.writeup-block,.answer{background:var(--paper);border:var(--line);border-radius:var(--r);box-shadow:var(--cut);margin:14px 0;padding:16px 18px}.block-title{font-size:.82rem;font-family:var(--text);letter-spacing:0;background:var(--ink);color:var(--paper);border-radius:2px;margin:0 0 10px;padding:1px 8px;font-weight:800;display:inline-block}.condition p:first-of-type,.step p:first-of-type{margin-top:0}.condition p:last-child,.step p:last-child{margin-bottom:0}.figure{text-align:center;margin:14px 0 0}.figure img{border:var(--line);background:#fff;border-radius:2px;max-width:100%;height:auto;display:inline-block}.figure figcaption{color:var(--ink-soft);margin-top:6px;font-size:.85rem}.step-figure img{object-fit:contain;width:100%;max-height:260px}.idea{border-left:4px solid var(--marker);margin:14px 0;padding-left:12px}.steps-head{justify-content:space-between;align-items:center;gap:8px;display:flex}.steps-head .block-title{margin-bottom:4px}.steps ol{counter-reset:step;margin:0;padding:0;list-style:none}.step{border-bottom:2px dashed var(--grid);counter-increment:step;padding:12px 0 12px 44px;position:relative}.step:last-child{border-bottom:0;padding-bottom:2px}.step:before{content:counter(step);width:30px;height:30px;font-family:var(--display);background:var(--marker);color:#1c2150;border:2px solid #1c2150;border-radius:2px;place-items:center;font-size:.9rem;font-weight:700;display:grid;position:absolute;top:12px;left:0}.step h3{margin-bottom:4px;font-size:1.02rem;line-height:1.35}.writeup-block{padding-left:10px;padding-right:10px}.writeup-block .block-title{margin-left:6px}.writeup-img{background:#fff;border-radius:2px;width:100%;max-width:560px;height:auto;margin:0;display:block}.writeup p{margin:.15rem 0;line-height:1.7}.writeup{overflow-x:auto}.answer{background:var(--marker);color:#1c2150;border-color:#1c2150}.answer .block-title{color:var(--marker);background:#1c2150}.answer-text{margin:0;font-size:1.2rem;font-weight:700}.subtasks{margin:0;padding:0;font-size:1.1rem;list-style:none}.subtasks li{padding:3px 0}.verified{color:#14774a;background:#fff;border:2px solid #1c2150;border-radius:2px;margin:12px 0 0;padding:2px 8px;font-size:.85rem;font-weight:700;display:inline-block}.verified:before{content:"✓ "}.report{color:var(--ink-soft);font-size:.92rem}button,.btn{font:inherit;cursor:pointer;text-align:center;border-radius:var(--r);background:var(--paper);color:var(--ink);border:var(--line);box-shadow:var(--cut-sm);padding:9px 14px;font-weight:700;text-decoration:none}.btn:hover,button:hover{background:var(--marker);color:#1c2150;text-decoration:none}.btn.primary,.next-step{background:var(--pen);color:#fff;border-color:var(--ink)}.btn.primary:hover,.next-step:hover{background:var(--pen);color:#fff;text-decoration:none}button:active{box-shadow:none;transform:translate(3px,3px)}.stepwise-toggle{box-shadow:none;padding:5px 10px;font-size:.85rem}.next-step{width:100%;margin-top:12px}.prev-next{grid-template-columns:1fr auto 1fr;gap:10px;margin:22px 0 8px;display:grid}.prev-next .primary{justify-self:end}[hidden]{display:none!important}.ad-slot{min-height:250px;margin:16px 0}.ad-slot.ad-top{min-height:100px}@media (width>=721px){.ad-slot.ad-top{min-height:90px}}.video-slot{aspect-ratio:16/9;border:var(--line);border-radius:var(--r);box-shadow:var(--cut);background:#000;margin:18px 0;overflow:hidden}.video-slot iframe{border:0;width:100%;height:100%}.katex{font-size:1.08em}.katex-display{padding:4px 0;overflow:auto hidden}.math-error{color:#c62828}@media (width<=520px){body{font-size:16px}h1{font-size:1.3rem}.logo span{font-size:.95rem}.header-row{gap:10px}.condition,.steps,.writeup-block,.answer{padding:14px}.writeup-block{padding:12px 6px 6px}.book-head{gap:12px}.book-cover.big{width:72px;height:96px}.prev-next{grid-template-columns:1fr 1fr}.prev-next .ghost{grid-area:2/1/auto/-1}}@media (prefers-reduced-motion:reduce){*{transform:none!important}}.grid-title{margin:18px 0 6px;font-weight:700}.grid-title a{color:var(--ink)}.section-list ul{columns:2 240px;column-gap:18px;margin:0;padding:0;font-size:.92rem;list-style:none}.section-list li{break-inside:avoid;padding:3px 0}.section-list a{color:var(--pen)}@media (prefers-color-scheme:dark){.writeup-img{filter:invert(.9)hue-rotate(180deg);background:#fff}}