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

body {
    font-family: 'Inter', 'Segoe UI', -apple-system, BlinkMacSystemFont, sans-serif;
    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 20px;
}

.calculator {
    width: 350px;
    background: #1e1e1e;
    border-radius: 20px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.5);
    overflow: hidden;
    animation: fadeIn 0.5s ease-in-out;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.display-area {
    background: #1e1e1e;
    padding: 30px 20px;
    min-height: 150px;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    gap: 5px;
}

.prev-expression {
    font-size: 14px;
    color: #888888;
    text-align: right;
    min-height: 20px;
    font-weight: 400;
}

.current-display {
    font-size: 48px;
    font-weight: 700;
    color: #ffffff;
    text-align: right;
    word-wrap: break-word;
    word-break: break-all;
    line-height: 1.2;
}

.buttons-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 2px;
    padding: 10px;
    background: #1e1e1e;
}

.btn {
    border: none;
    font-size: 24px;
    font-weight: 700;
    font-family: 'Segoe UI', 'Inter', sans-serif;
    padding: 25px;
    border-radius: 50%;
    cursor: pointer;
    transition: all 0.2s ease;
    user-select: none;
    -webkit-tap-highlight-color: transparent;
}

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

.btn-number {
    background: #333333;
    color: #ffffff;
}

.btn-number:hover {
    background: #4a4a4a;
}

.btn-operator {
    background: #ff9500;
    color: #ffffff;
}

.btn-operator:hover {
    background: #ffae42;
}

.btn-function {
    background: #a5a5a5;
    color: #000000;
}

.btn-function:hover {
    background: #c7c7c7;
}

.btn-zero {
    grid-column: span 2;
    border-radius: 50px;
}

/* Responsive Design */
@media (max-width: 400px) {
    .calculator {
        width: 100%;
        max-width: 350px;
    }
    
    .current-display {
        font-size: 40px;
    }
    
    .btn {
        padding: 20px;
        font-size: 20px;
    }
}

/* Accessibility */
.btn:focus {
    outline: 2px solid #ffffff;
    outline-offset: 2px;
}

.btn:focus:not(:focus-visible) {
    outline: none;
}