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

:root {
  --background: #eef2f7;
  --panel: #ffffff;
  --key: #e2e8f0;
  --key-hover: #cbd5e1;
  --key-active: #94a3b8;
  --key-border: #cbd5e1;
  --key-border-hover: #94a3b8;
  --key-border-active: #64748b;
  --ink: #0f172a;
  --display: #0f172a;
  --display-ink: #f8fafc;
  --focus: #2563eb;
}

body {
  margin: 0;
  min-height: 100vh;
  padding: 24px 16px;
  display: flex;
  align-items: flex-start;
  justify-content: center;
  background: var(--background);
  color: var(--ink);
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
}

.calculator {
  width: 100%;
  max-width: 420px;
  padding: 14px;
  background: var(--panel);
  border-radius: 20px;
  box-shadow: 0 12px 32px rgba(15, 23, 42, 0.14);
}

.display {
  padding: 12px 16px;
  border-radius: 14px;
  background: var(--display);
  color: var(--display-ink);
  font-size: clamp(30px, 8vw, 36px);
  font-weight: 600;
  line-height: 1.25;
  text-align: right;
  font-variant-numeric: tabular-nums;
  white-space: nowrap;
  overflow: hidden;
}

.keypad {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 10px;
  margin-top: 12px;
}

.key {
  font-family: inherit;
  font-size: 20px;
  font-weight: 600;
  color: var(--ink);
  background: var(--key);
  border: 1px solid var(--key-border);
  border-radius: 12px;
  min-height: 54px;
  padding: 8px 0;
  cursor: pointer;
  user-select: none;
  -webkit-user-select: none;
  touch-action: manipulation;
  -webkit-tap-highlight-color: transparent;
  transition: background-color 120ms ease, border-color 120ms ease, transform 120ms ease;
}

@media (hover: hover) and (pointer: fine) {
  .key:hover {
    background: var(--key-hover);
    border-color: var(--key-border-hover);
  }
}

.key:active {
  background: var(--key-active);
  border-color: var(--key-border-active);
  transform: scale(0.97);
}

.key:focus-visible {
  outline: 3px solid var(--focus);
  outline-offset: 2px;
}

.key-span-3 {
  grid-column: span 3;
}

.key-span-2 {
  grid-column: span 2;
}

@media (prefers-reduced-motion: reduce) {
  .key {
    transition: none;
  }

  .key:active {
    transform: none;
  }
}