回复：两个好解释，用手指头都能数。我就问你3个，4个，8个怎

Two legs is not very simple,24 cases.We use a1b2 express leg1 put on sock and leg2 put shoe....All cases are:a1a2b1b2,a1b1a2b2,......,b2b1a2a1.We cancel half for leg1 and cancel half for leg2.But for leg2 cancel half which half is canceled already.So,only 1/4 are left.