* { box-sizing: border-box; }
body { font-family: Arial, sans-serif; background: #f3f4f6; margin: 0; padding: 20px; }
.container { max-width: 800px; margin: 0 auto; }
header { text-align: center; margin-bottom: 20px; }
.card { background: white; padding: 20px; border-radius: 8px; box-shadow: 0 2px 6px rgba(0,0,0,0.08); }
.input-group { margin-bottom: 12px; }
.input-group label { display: block; font-weight: bold; margin-bottom: 6px; }
input[type="number"] { width: 100%; padding: 8px; border: 1px solid #ddd; border-radius: 4px; }
button { padding: 10px 14px; background: #2563eb; color: white; border: none; border-radius: 6px; cursor: pointer; }
button[disabled] { opacity: 0.6; cursor: not-allowed; }
.result { margin-top: 16px; }
.error { color: #b91c1c; }