:root {
  --bg: #0f172a;
  --card: #111827;
  --muted: #94a3b8;
  --text: #e2e8f0;
  --accent: #38bdf8;
  --accent-2: #22c55e;
  --danger: #ef4444;
  --border: #1f2937;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, sans-serif;
  background: radial-gradient(1200px 800px at 20% 10%, #1e293b 0%, #0b1020 60%);
  color: var(--text);
  min-height: 100vh;
  display: grid;
  place-items: center;
  padding: 24px;
}

.app {
  width: min(900px, 100%);
}

.app__header {
  text-align: center;
  margin-bottom: 20px;
}

.app__header h1 {
  margin: 0 0 8px 0;
  font-size: clamp(1.8rem, 2.6vw, 2.6rem);
}

.app__header p {
  margin: 10px 0 0 0;
  color: var(--muted);
}

.nav {
  display: flex;
  justify-content: center;
  gap: 14px;
  flex-wrap: wrap;
  margin-top: 6px;
}

.nav__link {
  text-decoration: none;
  color: var(--text);
  border: 1px solid var(--border);
  padding: 6px 10px;
  border-radius: 999px;
  font-size: 0.9rem;
}

.nav__link:hover {
  border-color: var(--accent);
  color: var(--accent);
}

.card {
  background: linear-gradient(180deg, rgba(17,24,39,.95), rgba(17,24,39,.9));
  border: 1px solid var(--border);
  border-radius: 16px;
  padding: clamp(18px, 3vw, 28px);
  box-shadow: 0 12px 30px rgba(0,0,0,.35);
}

.controls {
  display: grid;
  grid-template-columns: repeat(2, minmax(0,1fr));
  gap: 14px;
  margin-bottom: 16px;
}

.control {
  display: grid;
  gap: 6px;
}

select {
  background: #0b1222;
  border: 1px solid #1e293b;
  color: var(--text);
  padding: 10px 12px;
  border-radius: 10px;
}

.toggle {
  display: inline-flex;
  gap: 8px;
  align-items: center;
}

.timer {
  margin-top: 6px;
  font-weight: 700;
  font-size: 1.2rem;
  color: var(--accent);
}

.problem__row {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
  margin-bottom: 16px;
}

.label {
  color: var(--muted);
  font-size: 0.9rem;
  margin-bottom: 6px;
}

.value {
  font-size: 1.2rem;
  font-weight: 600;
  letter-spacing: 0.5px;
}

.mono {
  font-variant-numeric: tabular-nums;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
}

.form {
  display: grid;
  gap: 14px;
}

.form__row {
  display: grid;
  gap: 6px;
}

input, textarea {
  background: #0b1222;
  border: 1px solid #1e293b;
  color: var(--text);
  padding: 12px 14px;
  border-radius: 10px;
  font-size: 1rem;
}

input:focus, textarea:focus {
  outline: 2px solid var(--accent);
  border-color: transparent;
}

.form__actions {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin-top: 4px;
}

.btn {
  border: none;
  border-radius: 10px;
  padding: 10px 14px;
  font-weight: 600;
  cursor: pointer;
}

.btn--primary {
  background: var(--accent);
  color: #00111a;
}

.btn--ghost {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text);
}

.result {
  margin-top: 16px;
  padding: 12px 14px;
  border-radius: 10px;
  border: 1px dashed #334155;
  background: rgba(2,6,23,.4);
  min-height: 48px;
}

.result.success { border-color: var(--accent-2); }
.result.error { border-color: var(--danger); }

.explanation {
  margin-top: 10px;
  padding: 12px 14px;
  border-radius: 10px;
  border: 1px solid #1e293b;
  background: rgba(2,6,23,.35);
  color: var(--muted);
  min-height: 48px;
}

.explain-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0,1fr));
  gap: 12px;
}

.explain-item {
  background: rgba(11, 18, 34, 0.6);
  border: 1px solid #1e293b;
  border-radius: 10px;
  padding: 10px 12px;
}

.explain-text {
  margin-top: 10px;
  color: var(--muted);
  line-height: 1.45;
}

.score {
  margin-top: 16px;
  display: grid;
  grid-template-columns: repeat(3, minmax(0,1fr));
  gap: 12px;
}

.score__item {
  background: #0b1222;
  border: 1px solid #1e293b;
  border-radius: 10px;
  padding: 12px;
  text-align: center;
}

.app__footer {
  text-align: center;
  margin-top: 16px;
  color: var(--muted);
}

@media (max-width: 640px) {
  .problem__row, .score, .controls, .explain-grid {
    grid-template-columns: 1fr;
  }
}