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

body {
    background: linear-gradient(135deg, #1a1a1a 0%, #2d2d2d 100%);
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    color: #fff;
}

.container {
    padding: 20px;
}

.calculator {
    background: #000;
    border-radius: 30px;
    padding: 20px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.9);
    width: 320px;
}

.display {
    margin-bottom: 20px;
    background: #1c1c1c;
    border-radius: 15px;
    padding: 10px;
}

.display-input {
    width: 100%;
    background: #1c1c1c;
    border: none;
    color: #fff;
    font-size: 48px;
    text-align: right;
    padding: 20px;
    outline: none;
    font-weight: 300;
    border-radius: 10px;
}

.buttons {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.row {
    display: flex;
    gap: 12px;
    justify-content: space-between;
}

.btn {
    flex: 1;
    padding: 20px;
    border: none;
    border-radius: 15px;
    font-size: 24px;
    font-weight: 600;
    cursor: pointer;
    background: #333;
    color: #fff;
    transition: all 0.2s ease;
}

.btn:active {
    transform: scale(0.95);
    opacity: 0.8;
}

.btn-operator {
    background: #ff9500;
    color: #fff;
    font-weight: 700;
}

.btn-operator:active {
    background: #ffb143;
}

.btn-function {
    background: #555;
    color: #fff;
}

.btn-function:active {
    background: #777;
}

.btn-equals {
    background: #ff9500;
    color: #fff;
    font-weight: 700;
    flex: 2 !important;
}

.btn-equals:active {
    background: #ffb143;
}

/* Responsive design */
@media (max-width: 400px) {
    .calculator {
        width: 100%;
        max-width: 320px;
    }

    .btn {
        font-size: 20px;
        padding: 16px;
    }

    .display-input {
        font-size: 36px;
        padding: 16px;
    }
}