* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body {
  background: #f5f7fa;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
}

.container {
  max-width: 400px;
  width: 100%;
  background: #ffffff;
  border-radius: 12px;
  box-shadow: 0 8px 16px rgba(0,0,0,0.1);
  padding: 20px;
  text-align: center;
}

h1 {
  margin-bottom: 20px;
  color: #333;
}

.calculator {
  width: 100%;
}

#display {
  width: 100%;
  height: 60px;
  background: linear-gradient(145deg, #e8eaf0, #ffffff);
  border: none;
  border-radius: 8px;
  box-shadow: inset 4px 4px 8px rgba(0,0,0,0.05),
              inset -4px -4px 8px rgba(255,255,255,0.7);
  font-size: 2rem;
  text-align: right;
  padding: 0 15px;
  margin-bottom: 15px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

button {
  font-size: 1.2rem;
  padding: 15px;
  border: none;
  border-radius: 8px;
  background: linear-gradient(145deg, #ffffff, #e0e0e0);
  box-shadow: 4px 4px 8px rgba(0,0,0,0.1),
              -4px -4px 8px rgba(255,255,255,0.7);
  cursor: pointer;
  transition: all 0.2s ease;
}

button:hover {
  transform: translateY(-2px);
  box-shadow: 6px 6px 12px rgba(0,0,0,0.1),
              -6px -6px 12px rgba(255,255,255,0.7);
}

button:active {
  transform: translateY(2px);
  box-shadow: inset 4px 4px 8px rgba(0,0,0,0.1),
              inset -4px -4px 8px rgba(255,255,255,0.7);
}

button.equals {
  grid-row: span 2;
  background: #4e54c8;
  color: #fff;
}

button.zero {
  grid-column: span 2;
}

footer {
  margin-top: 20px;
  font-size: 0.9rem;
  color: #777;
}