:root {
  --bg: #f8fafc;
  --panel: #ffffff;
  --text: #0f172a;
  --muted: #475569;
  --accent: #16a34a;
  --danger: #dc2626;
  --brand: #2563eb;
  --border: #e2e8f0;
  --shadow: 0 10px 20px rgba(2, 6, 23, 0.06), 0 2px 6px rgba(2, 6, 23, 0.05);
}

* { box-sizing: border-box; }
html, body { height: 100%; }
body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen,
    Ubuntu, Cantarell, 'Fira Sans', 'Droid Sans', 'Helvetica Neue', Arial, sans-serif;
  background: radial-gradient(1200px 600px at 10% 0%, #ffffff 0%, #f8fafc 70%);
  color: var(--text);
}

.app { max-width: 900px; margin: 0 auto; padding: 24px; }

h1 { margin: 8px 0 20px; font-size: 28px; font-weight: 800; color: #0f172a; }

.top-bar { display: flex; gap: 12px; align-items: center; }
.timer, .progress { background: var(--panel); padding: 10px 14px; border-radius: 10px; color: var(--muted); border: 1px solid var(--border); box-shadow: var(--shadow); }
.start { margin-left: auto; padding: 10px 16px; border-radius: 10px; border: 1px solid var(--border); background: var(--panel); color: #0f172a; cursor: pointer; box-shadow: var(--shadow); }
.start:hover { background: #f1f5f9; }
.start:disabled { opacity: 0.6; cursor: not-allowed; }

.status { min-height: 22px; margin: 10px 0 8px; color: var(--muted); }

.game { background: var(--panel); border: 1px solid var(--border); border-radius: 16px; padding: 28px; box-shadow: var(--shadow); }
.problem { text-align: center; font-size: 72px; font-weight: 900; letter-spacing: 1px; margin: 10px 0 24px; color: #0f172a; }
.options { display: grid; grid-template-columns: repeat(2, 1fr); gap: 16px; }

.option-btn { font-size: 28px; font-weight: 800; padding: 22px; border-radius: 14px; border: 1px solid var(--border); background: #ffffff;
  color: #0f172a; cursor: pointer; transition: transform 80ms ease, background 0.2s ease, box-shadow 0.2s ease; box-shadow: var(--shadow); }
.option-btn:hover { transform: translateY(-1px); background: #f8fafc; }
.option-btn:active { transform: translateY(0); }
.option-btn:disabled { opacity: 0.8; cursor: default; transform: none; }
.option-btn.correct { outline: 2px solid var(--accent); box-shadow: 0 0 0 6px rgba(22,163,74,0.12), var(--shadow); animation: pop 280ms ease-out; }
.option-btn.wrong { outline: 2px solid var(--danger); box-shadow: 0 0 0 6px rgba(220,38,38,0.12), var(--shadow); animation: shake 300ms ease-in-out; }

.result { margin-top: 18px; padding: 18px; background: var(--panel); border: 1px solid var(--border); border-radius: 14px; box-shadow: var(--shadow); }
.result .score { font-size: 20px; margin-bottom: 8px; }
.again { padding: 10px 16px; border-radius: 10px; border: 1px solid var(--border); background: var(--panel); color: #0f172a; cursor: pointer; box-shadow: var(--shadow); }
.again:hover { background: #f1f5f9; }

.hidden { display: none; }

/* Animations */
@keyframes pop {
  0% { transform: scale(1); }
  50% { transform: scale(1.08); }
  100% { transform: scale(1); }
}

@keyframes shake {
  0% { transform: translateX(0); }
  20% { transform: translateX(-6px); }
  40% { transform: translateX(6px); }
  60% { transform: translateX(-4px); }
  80% { transform: translateX(4px); }
  100% { transform: translateX(0); }
}

.fx-layer { position: fixed; inset: 0; pointer-events: none; overflow: hidden; z-index: 999; }
.confetti { position: absolute; width: 8px; height: 14px; border-radius: 2px; will-change: transform, opacity; transform: translate(-50%, -50%); }
@keyframes confetti {
  0% { opacity: 1; transform: translate(0, 0) rotate(0deg); }
  100% { opacity: 0; transform: translate(var(--tx), var(--ty)) rotate(var(--rot)); }
}

@media (max-width: 640px) {
  .problem { font-size: 52px; }
  .option-btn { font-size: 22px; padding: 18px; }
}