S=input()
li=[0]*26
li2=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]
for i in range(len(S)):
  for j in range(len(S)):
    s=len(set(S[i:j+1]))
    if s>=1:
      li[s-1]+=1
com=[x*y for (x,y) in zip(li, li2)]
print(sum(com)/sum(li))