from bisect import *

S = list(input())
N = len(S)
D = [[0, N+1] for i in range(26)]
for i in range(N):
    D[ord(S[i]) - ord("a")].append(i + 1)

for i in range(26):
    D[i].sort()    

ans = 0
for i in range(N):
    n = ord(S[i]) - ord("a")
    right = bisect_right(D[n], i+1)
    left = bisect_left(D[n], i+1)-1
    ans += (N+1 - i-1)*(i+1 - D[n][left])
    
print(ans/(N * (N + 1)//2))