* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: Arial, sans-serif;
}

body {
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: #f1f5f9;
}

.calculator {
  width: 320px;
  background: #0f172a;
  border-radius: 14px;
  padding: 18px;
  box-shadow: 0 16px 34px rgba(15, 23, 42, 0.35);
}

.calculator h1 {
  color: #e2e8f0;
  font-size: 1.2rem;
  margin-bottom: 12px;
}

.display {
  width: 100%;
  height: 62px;
  border: none;
  border-radius: 10px;
  margin-bottom: 12px;
  padding: 0 14px;
  text-align: right;
  font-size: 2rem;
  color: #0f172a;
  background: #e2e8f0;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 8px;
}

.btn {
  border: none;
  border-radius: 10px;
  height: 54px;
  cursor: pointer;
  font-size: 1.2rem;
  color: #e2e8f0;
  background: #1e293b;
  transition: 0.15s ease;
}

.btn:hover {
  filter: brightness(1.15);
}

.operator {
  background: #334155;
}

.action {
  background: #475569;
}

.equals {
  background: #0ea5e9;
  color: #082f49;
  font-weight: 700;
}

.zero {
  grid-column: span 2;
}