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

#menu {
    list-style: none;
    background: blue;
    display: flex;
}

#menu > li {
    position: relative;
}

#menu li a {
    display: block;
    padding: 12px 20px;
    color: white;
    text-decoration: none;
    white-space: nowrap;
}

#menu li a:hover {
    background: darkblue;
}

#menu li ul {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    background: blue;
    list-style: none;
    min-width: 200px;
}

#menu li ul li {
    position: relative;
}

#menu li ul li ul {
    top: 0;
    left: 100%;
}

#menu li:hover > ul {
    display: block;
}