最后剩桃子数为:(1-1/5){(1-1/5)[(1-1/5)M-1]-1}-1
(1-1/5)*{(1-1/5)*[(1-1/5)*M-1]-1}-1
4/5((M4/5-1)4/5-1)-1=========64/225M-51/25
m-{[m-(m-1/5m+1)*1/5+1]*1/5+1}=104/125m+31/25