First time working with Variable and Loops in SCSS

I'm trying to assign a different left padding to each LI in a UL, but i'm not assign the variable correctly i dont believe and it wont let me write the each statement without using variables..

Here is what i have, can anyone point me in the right direction, thanks in advance

$menu-li: .menu li;
$menu-ul: .menu ul;
.menu li{
  width:30%;
  padding: 10px;

}
@each $menu-li in $menu-ul{
  padding-left: random(20)+px;
}