#menu-dung, #menu-dung ul {
    list-style-type: none;
    padding: 0;
    margin: 0;
    width: 150px;
}

#menu-dung li {
    background-color: #000000;
    position: relative;
    border-bottom: 1px solid #FFF;
}

#menu-dung li a {
    display: block;
    color: #FFF;
    text-decoration: none;
    padding: 10px;
}

#menu-dung li:hover > a {
    background-color: darkgray;
}

.sub-menu, .sub-menu-2 {
    display: none;
    position: absolute;
    left: 150px;
    top: 0;
    width: max-content;
}

#menu-dung li:hover > .sub-menu {
    display: block;
}

.sub-menu li:hover > .sub-menu-2 {
    display: block;
}