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

body {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: #1c1c1e;
  font-family: -apple-system, "Helvetica Neue", Arial, sans-serif;
}

.calculator {
  width: 320px;
  padding: 16px;
  border-radius: 24px;
  background: #2c2c2e;
}

.display {
  height: 96px;
  padding: 0 16px;
  display: flex;
  align-items: flex-end;
  justify-content: flex-end;
  color: #fff;
  font-size: 56px;
  font-weight: 300;
  overflow: hidden;
  white-space: nowrap;
}

.display.error {
  font-size: 40px;
  color: #ff9f0a;
}

.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
  margin-top: 16px;
}

.key {
  height: 68px;
  border: none;
  border-radius: 50%;
  font-size: 26px;
  color: #fff;
  background: #505050;
  cursor: pointer;
  transition: filter 0.1s ease;
}

.key:hover { filter: brightness(1.2); }
.key:active { filter: brightness(1.45); }

.key:focus-visible {
  outline: 2px solid #0a84ff;
  outline-offset: 2px;
}

.key-fn {
  background: #a5a5a5;
  color: #000;
}

.key-op,
.key-eq {
  background: #ff9f0a;
  font-size: 30px;
}

.key-zero {
  grid-column: span 2;
  border-radius: 40px;
  text-align: left;
  padding-left: 26px;
}

@media (max-width: 360px) {
  .calculator { width: 100%; border-radius: 0; }
}