*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

:root {
  --bg: #f0f4ff;
  --card: #ffffff;
  --primary: #4f6ef7;
  --primary-dark: #3a56d4;
  --accent: #ff6b6b;
  --text: #1a1a2e;
  --muted: #6b7280;
  --radius: 12px;
  --shadow: 0 4px 20px rgba(79, 110, 247, 0.12);

  /* Group colors */
  --c1: #ff6b6b;
  --c2: #ff9f43;
  --c3: #ffd32a;
  --c4: #0be881;
  --c5: #00d2d3;
  --c6: #48dbfb;
  --c7: #a29bfe;
  --c8: #fd79a8;
  --c9: #6c5ce7;
  --c10: #e17055;
  --c11: #00b894;
  --c12: #74b9ff;
}

body {
  background: var(--bg);
  color: var(--text);
  font-family: 'Segoe UI', system-ui, sans-serif;
  min-height: 100vh;
  display: flex;
  align-items: flex-start;
  justify-content: center;
  padding: 2rem 1rem;
}

.app {
  width: 100%;
  max-width: 800px;
  display: flex;
  flex-direction: column;
  gap: 2rem;
}

header {
  text-align: center;
}

h1 {
  font-size: 2.5rem;
  font-weight: 800;
  color: var(--primary);
  letter-spacing: -1px;
}

.subtitle {
  color: var(--muted);
  margin-top: 0.25rem;
  font-size: 1rem;
}

/* Controls */
.controls {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.5rem 2rem;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 2rem;
  flex-wrap: wrap;
}

.input-group {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.5rem;
}

.input-group label {
  font-size: 0.8rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--muted);
}

.input-row {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.stepper {
  width: 36px;
  height: 36px;
  border: none;
  border-radius: 8px;
  background: var(--bg);
  color: var(--primary);
  font-size: 1.2rem;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.15s, transform 0.1s;
  display: flex;
  align-items: center;
  justify-content: center;
}

.stepper:hover {
  background: var(--primary);
  color: white;
}

.stepper:active {
  transform: scale(0.92);
}

input[type="number"] {
  width: 64px;
  height: 48px;
  border: 2px solid #e5e7eb;
  border-radius: 8px;
  font-size: 1.5rem;
  font-weight: 700;
  text-align: center;
  color: var(--text);
  background: white;
  transition: border-color 0.15s;
  -moz-appearance: textfield;
}

input[type="number"]:focus {
  outline: none;
  border-color: var(--primary);
}

input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
  -webkit-appearance: none;
}

.operator {
  font-size: 2rem;
  font-weight: 700;
  color: var(--muted);
  padding-top: 1.2rem;
}

/* Equation */
.equation {
  text-align: center;
  font-size: 1.8rem;
  font-weight: 700;
  color: var(--text);
  min-height: 2.5rem;
}

.equation .result {
  color: var(--primary);
}

/* Visualization */
.visualization {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  justify-content: flex-start;
}

.group-card {
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1rem 1.25rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.75rem;
  animation: pop 0.2s ease-out;
  border-top: 4px solid var(--group-color, var(--primary));
  min-width: 80px;
}

@keyframes pop {
  from { transform: scale(0.85); opacity: 0; }
  to   { transform: scale(1);    opacity: 1; }
}

.group-label {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--group-color, var(--primary));
}

.items-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  justify-content: center;
  max-width: 160px;
}

.item {
  width: 28px;
  height: 28px;
  border-radius: 6px;
  background: var(--group-color, var(--primary));
  opacity: 0.85;
  transition: transform 0.1s;
}

.item:hover {
  transform: scale(1.15);
  opacity: 1;
}

footer {
  text-align: center;
  color: var(--muted);
  font-size: 0.85rem;
  padding-bottom: 1rem;
}

@media (max-width: 480px) {
  h1 { font-size: 2rem; }
  .controls { gap: 1rem; padding: 1rem; }
  .equation { font-size: 1.4rem; }
}