body {
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    margin: 0;
    font-family: 'Arial', sans-serif;
    background-color: #f0f0f0;
    color: #333;
}

.container {
    background-color: #fff;
    padding: 40px;
    border-radius: 10px;
    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
    text-align: center;
}

.multiplication-problem {
    font-size: 3em;
    margin-bottom: 30px;
    color: #007bff;
    min-height: 1.5em; /* コンテンツがないときに高さが変化しないように */
    display: flex;
    justify-content: center;
    align-items: center;
}

button {
    padding: 15px 30px;
    font-size: 1.2em;
    background-color: #28a745;
    color: white;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    transition: background-color 0.3s ease;
}

button:hover {
    background-color: #218838;
}

button:disabled {
    background-color: #cccccc;
    cursor: not-allowed;
}