body {
    font-family: Arial, sans-serif;
    background: #f3f4f6;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
}

.calculator {
    width: 320px;
    background: white;
    padding: 20px;
    border-radius: 12px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
}

.display {
    background: #1f2937;
    color: white;
    font-size: 2.5rem;
    padding: 15px;
    border-radius: 8px;
    text-align: right;
    margin-bottom: 20px;
}

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

.btn {
    background: #e5e7eb;
    border: none;
    padding: 18px;
    font-size: 1.3rem;
    border-radius: 8px;
    cursor: pointer;
    transition: 0.2s;
}

.btn:hover {
    background: #d1d5db;
}

.op {
    background: #fcd34d;
}

.op:hover {
    background: #fbbf24;
}

.clear {
    grid-column: span 2;
    background: #ef4444;
    color: white;
}

.equal {
    grid-column: span 2;
    background: #10b981;
    color: white;
}