In a regular hexagon ABCDEF, ¯¯¯¯¯¯¯¯AB+¯¯¯¯¯¯¯¯AC+¯¯¯¯¯¯¯¯¯AD+¯¯¯¯¯¯¯¯AE+¯¯¯¯¯¯¯¯AF=k¯¯¯¯¯¯¯¯¯AD, then k is equal to