body {
    font-family: "Helvetica Neue",
        Arial,
        "Hiragino Kaku Gothic ProN",
        "Hiragino Sans",
        Meiryo,
        sans-serif;
    margin: 0;
    background-color: #fff;
}
header {
    margin-top: 32px;
    text-align: center;
}
h1 {
    font-weight: normal;
    font-size: 16px;
}
main {
    width: 220px;
    margin: 64px auto;
    padding: 0;
}

.formula {
    font-size: 48px;
    margin-bottom: 64px;
}


button {
    font-size: 16px;
    padding: 0;
    border: 2px solid #444;
    color: #444;
    width: 80px;
    height: 80px;
    line-height: 80px;
    border-radius: 40px;
    text-align: center;
    background-color: #fff;
    margin: 0 auto;
    display: block;
}
button:hover {
    opacity: 0.6;
    cursor: pointer;
}
.hidden {
    display: none;
}
#buttonShowAnswer {
    border-color: #0077C7;
    color:  #0077C7;
}