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

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

/* Container */
.container {
    background: white;
    border-radius: 20px;
    box-shadow: 0 20px 60px rgba(0,0,0,0.3);
    padding: 30px;
    max-width: 500px;
    width: 100%;
}

/* Header */
.header {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 15px;
    margin-bottom: 20px;
}

.logo {
    width: 40px;
    height: 40px;
    display: none; /* Hidden by default, shows when logo exists */
}

.logo.visible {
    display: block;
}

h1 {
    text-align: center;
    color: #333;
    font-size: 28px;
}

/* Display */
.memory-display {
    font-size: 14px;
    color: #666;
    text-align: right;
    margin-bottom: 10px;
    min-height: 20px;
}

.display {
    background: #f5f5f5;
    padding: 20px;
    border-radius: 10px;
    margin-bottom: 20px;
    font-size: 32px;
    text-align: right;
    font-weight: bold;
    color: #333;
    border: 2px solid #ddd;
    min-height: 70px;
    word-wrap: break-word;
}

/* Button Grid */
.button-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    margin-bottom: 20px;
}

/* Base Button Styles */
button {
    padding: 20px;
    font-size: 18px;
    border: none;
    border-radius: 10px;
    cursor: pointer;
    transition: all 0.2s;
    font-weight: 600;
}

button:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 8px rgba(0,0,0,0.2);
}

button:active {
    transform: translateY(0);
}

/* Number Buttons */
.btn-number {
    background: #e9ecef;
    color: #333;
}

.btn-number:hover {
    background: #dee2e6;
}

/* Operator Buttons */
.btn-operator {
    background: #667eea;
    color: white;
}

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

/* Equals Button */
.btn-equals {
    background: #28a745;
    color: white;
    grid-column: span 2;
}

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

/* Clear Button */
.btn-clear {
    background: #dc3545;
    color: white;
}

.btn-clear:hover {
    background: #c82333;
}

/* Memory Buttons */
.btn-memory {
    background: #ffc107;
    color: #333;
}

.btn-memory:hover {
    background: #e0a800;
}

/* Factorial Button */
.btn-factorial {
    width: 100%;
    margin-bottom: 10px;
}

/* History */
.history {
    margin-top: 20px;
    padding: 15px;
    background: #f8f9fa;
    border-radius: 10px;
    max-height: 200px;
    overflow-y: auto;
}

.history h3 {
    font-size: 16px;
    margin-bottom: 10px;
    color: #666;
}

.history-item {
    padding: 8px;
    background: white;
    margin-bottom: 5px;
    border-radius: 5px;
    font-size: 14px;
    color: #333;
}

.btn-clear-history {
    width: 100%;
    margin-top: 10px;
    padding: 10px;
}

/* Error State */
.error {
    color: #dc3545;
    font-size: 18px;
}

/* Info */
.info {
    text-align: center;
    margin-top: 20px;
    color: #666;
    font-size: 14px;
}

/* Responsive Design */
@media (max-width: 600px) {
    .container {
        padding: 20px;
    }
    
    h1 {
        font-size: 24px;
    }
    
    .display {
        font-size: 28px;
        padding: 15px;
    }
    
    button {
        padding: 15px;
        font-size: 16px;
    }
}