body {
  background: #f0f2f5;
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}

.calculator {
  background: #ffffff;
  padding: 20px;
  border-radius: 12px;
  box-shadow: 0 8px 16px rgba(0, 0, 0, 0.1);
  width: 320px;
  max-width: 90%;
}

.display {
  background: #e0e0e0;
  padding: 20px;
  text-align: right;
  font-size: 2rem;
  border-radius: 8px;
  margin-bottom: 15px;
  min-height: 60px;
}

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

button {
  padding: 20px;
  font-size: 1.2rem;
  border: none;
  border-radius: 8px;
  background-color: #f9f9f9;
  cursor: pointer;
  transition: background 0.2s;
}

button:hover {
  background-color: #e0e0e0;
}

button:active {
  background-color: #d0d0d0;
}