* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Courier New', monospace;
}

body {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    background: #ff6b6b;
    padding: 20px;
}

.back-button {
    text-decoration: none;
    padding: 12px 20px;
    border: 3px solid #000;
    background: #fff;
    color: #000;
    font-weight: bold;
    box-shadow: 4px 4px 0 #000;
    margin-bottom: 20px;
    font-size: 1.2rem;
    transition: transform 0.1s;
}

.back-button:hover {
    background: #f8f9fa;
}

.back-button:active {
    transform: translate(4px, 4px);
    box-shadow: none;
}

.calculator {
    padding: 25px;
    border: 4px solid #000;
    background: #fff;
    box-shadow: 8px 8px 0 #000;
}

.display {
    padding: 20px;
    margin-bottom: 20px;
    border: 3px solid #000;
    background: #e9ecef;
    text-align: right;
    min-height: 100px;
}

.previous-operand {
    font-size: 1.2rem;
    color: #495057;
    min-height: 24px;
    font-weight: bold;
}

.current-operand {
    font-size: 2.5rem;
    color: #000;
    margin-top: 10px;
    font-weight: bold;
}

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

button {
    padding: 20px;
    font-size: 1.5rem;
    border: 3px solid #000;
    background: #fff;
    cursor: pointer;
    color: #000;
    font-weight: bold;
    transition: transform 0.1s;
    box-shadow: 4px 4px 0 #000;
}

button:active {
    transform: translate(4px, 4px);
    box-shadow: none;
}

.operator {
    background: #4c6ef5;
    color: #fff;
}

.equals {
    grid-column: span 2;
    background: #40c057;
    color: #fff;
}

.equals:active {
    background: #2b8a3e;
}

button:hover {
    background: #f8f9fa;
}

.operator:hover {
    background: #364fc7;
}

.equals:hover {
    background: #37b24d;
}