* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: linear-gradient(135deg, #1a1a2e 0%, #16213e 50%, #0f3460 100%);
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}
.calc-wrapper {
  padding: 1rem;
}
.calculator {
  background: #1e1e2e;
  border-radius: 20px;
  padding: 1.5rem;
  width: 320px;
  box-shadow:
    0 25px 60px rgba(0, 0, 0, 0.5),
    0 0 0 1px rgba(255, 255, 255, 0.05);
}
.display {
  background: #13131f;
  border-radius: 12px;
  padding: 1rem 1.2rem;
  margin-bottom: 1.2rem;
  text-align: right;
  min-height: 90px;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  gap: 0.3rem;
  overflow: hidden;
}
.display-expression {
  font-size: 0.85rem;
  color: #6b7280;
  min-height: 1.2em;
  word-break: break-all;
}
.display-result {
  font-size: 2.2rem;
  color: #f8f8f2;
  font-weight: 300;
  word-break: break-all;
  transition: font-size 0.1s;
}
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.6rem;
}
.btn {
  border: none;
  border-radius: 12px;
  font-size: 1.1rem;
  font-weight: 500;
  padding: 1rem 0;
  cursor: pointer;
  transition: transform 0.1s, filter 0.1s, box-shadow 0.1s;
  user-select: none;
}
.btn:active {
  transform: scale(0.93);
  filter: brightness(0.85);
}
.btn-number {
  background: #2a2a3e;
  color: #f8f8f2;
  box-shadow: 0 4px 0 #1a1a2e;
}
.btn-number:hover {
  background: #33334d;
}
.btn-operator {
  background: #3a3a5c;
  color: #bd93f9;
  box-shadow: 0 4px 0 #252540;
}
.btn-operator:hover {
  background: #44446e;
}
.btn-clear {
  background: #3d2a2a;
  color: #ff6b6b;
  box-shadow: 0 4px 0 #2a1a1a;
}
.btn-clear:hover {
  background: #4d3333;
}
.btn-equals {
  background: linear-gradient(135deg, #6c63ff, #4a6cf7);
  color: #fff;
  font-size: 1.3rem;
  box-shadow: 0 4px 0 #3a3aaa;
}
.btn-equals:hover {
  filter: brightness(1.1);
}
.btn-zero {
  grid-column: span 2;
}