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

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

.calculator {
    background: white;
    border-radius: 20px;
    box-shadow: 0 10px 40px rgba(0, 0, 0, 0.3);
    overflow: hidden;
    width: 320px;
}

.display {
    background: #2d3436;
    padding: 20px;
}

#display {
    width: 100%;
    background: #2d3436;
    border: none;
    color: #00ff00;
    font-size: 32px;
    text-align: right;
    padding: 10px;
    font-weight: bold;
    font-family: 'Courier New', monospace;
}

#display:focus {
    outline: none;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 1px;
    background: #dfe6e9;
    padding: 1px;
}

.btn {
    padding: 25px;
    font-size: 20px;
    font-weight: 600;
    border: none;
    background: white;
    cursor: pointer;
    transition: all 0.2s ease;
}

.btn:active {
    transform: scale(0.95);
}

.btn:hover {
    background: #f0f0f0;
}

.operator-btn {
    background: #667eea;
    color: white;
    font-weight: bold;
}

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

.function-btn {
    background: #ff7675;
    color: white;
    font-weight: bold;
}

.function-btn:hover {
    background: #e85a57;
}

.equals-btn {
    background: #00b894;
    color: white;
    font-weight: bold;
    grid-column: span 2;
}

.equals-btn:hover {
    background: #009c7f;
}

.zero {
    grid-column: span 2;
}

@media (max-width: 400px) {
    .calculator {
        width: 100%;
        height: 100vh;
        border-radius: 0;
    }

    .btn {
        padding: 20px;
        font-size: 18px;
    }

    #display {
        font-size: 28px;
    }
}