* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
}

.calculator {
    background: #fff;
    padding: 30px;
    border-radius: 20px;
    box-shadow: 0 10px 50px rgba(0, 0, 0, 0.3);
    width: 100%;
    max-width: 400px;
}

.calculator h1 {
    text-align: center;
    margin-bottom: 20px;
    color: #333;
    font-size: 24px;
}

#display {
    width: 100%;
    height: 80px;
    font-size: 32px;
    text-align: right;
    padding: 20px;
    margin-bottom: 20px;
    border: 2px solid #e0e0e0;
    border-radius: 10px;
    background: #f9f9f9;
    color: #333;
    font-weight: 500;
    cursor: default;
}

#display:focus {
    outline: none;
    border-color: #667eea;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 12px;
}

.btn {
    height: 70px;
    font-size: 24px;
    border: none;
    border-radius: 10px;
    background: #f0f0f0;
    color: #333;
    cursor: pointer;
    transition: all 0.2s ease;
    font-weight: 600;
}

.btn:hover {
    background: #e0e0e0;
    transform: translateY(-2px);
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
}

.btn:active {
    transform: translateY(0);
    box-shadow: 0 2px 6px rgba(0, 0, 0, 0.15);
}

.btn.operator {
    background: #667eea;
    color: #fff;
}

.btn.operator:hover {
    background: #5568d3;
}

.btn.equals {
    background: #28a745;
    color: #fff;
}

.btn.equals:hover {
    background: #218838;
}

.btn.clear {
    background: #ff6b6b;
    color: #fff;
}

.btn.clear:hover {
    background: #ee5a52;
}

.btn.delete {
    background: #ffa500;
    color: #fff;
}

.btn.delete:hover {
    background: #ff8c00;
}

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