s=input()
l=len(s)
pos={}
alpha='abcdefghijklmnopqrstuvwxyz'
for char in alpha:
    pos[char]=-1
ans=0
dp=0
for i in range(l):
    dp+=i-pos[s[i]]
    ans+=dp
    pos[s[i]]=i
ans/=(l*(l+1))//2
print(ans)