body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
  background-color: #f4f4f4;
  font-family: Arial, sans-serif;
}

#calculator {
  width: 270px;
  background-color: #222;
  padding: 20px;
  border-radius: 20px;
  box-shadow: 0 10px 20px rgba(0, 0, 0, 0.3);
}

#display {
  width: 100%;
  padding: 15px;
  font-size: 24px;
  margin-bottom: 15px;
  border: none;
  border-radius: 10px;
  text-align: right;
  background-color: #333;
  color: #fff;
  box-sizing: border-box;
}

#keys {
  display: grid;
  grid-template-columns: repeat(4, 60px);
  grid-gap: 10px;
  justify-content: center;
}

button {
  padding: 20px;
  font-size: 20px;
  border: none;
  border-radius: 50%;
  cursor: pointer;
  background-color: #444;
  color: white;
  transition: background-color 0.2s;
}

button:hover {
  background-color: #555;
}

.operator-btn {
  background-color: orange;
  color: white;
}

@media (max-width: 400px) {
  #calculator {
    width: 90%;
    padding: 15px;
  }

  #keys {
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 8px;
  }

  button {
    padding: 15px;
    font-size: 18px;
  }

  #display {
    font-size: 20px;
    padding: 12px;
  }
}