from collections import defaultdict

S = input()
N = len(S)
dp = [0] * (N+1)
appear_dict = defaultdict(list)

for i in range(N):
    if len(appear_dict[S[i]]) == 0:
        dp[i+1] = dp[i] + i + 1
    else:
        dp[i+1] = dp[i] + i - appear_dict[S[i]][-1]
    
    appear_dict[S[i]].append(i)

ans = sum(dp) / (N*(N+1)/2)
print(ans)