from collections import deque
s=list(map(lambda x:ord(x)-97,list(input())));n=len(s)
x=[deque() for i in range(26)]
for i in range(n):
    x[s[i]].append(i)
ans=0
for i in range(n):
    for j in range(26):
        if x[j]:
            ans+=n-x[j][0]
    x[s[i]].popleft()
print(ans/(n*(n+1)//2))