CSS expanding vertical menu

Sub Menu 1.1
Sub Menu 1.2
Sub Menu 1.3

Menu 2

Sub Menu 2.1
Sub Menu 2.2

Menu 3

Sub Menu 3.1
Sub Menu 3.1
Sub Menu 3.1
Sub Menu 3.1
Sub Menu 3.1
Sub Menu 3.1

Menu 4

Sub Menu 4.1
Sub Menu 4.1