from bisect import bisect_left
S = input().strip()
N = len(S)
S = "0"+S
D = {}
for i in range(1,N+1):
    if S[i] not in D:
        D[S[i]] = [0]
    D[S[i]].append(i)
dp = [0]*(N+1)
dp[1] = 1
if N>=2:
    if S[1]==S[2]:
        dp[2] = 3
    else:
        dp[2] = 4
for i in range(3,N+1):
    ind = bisect_left(D[S[i]],i)
    dp[i] = 2*dp[i-1]-dp[i-2]+i-D[S[i]][ind-1]
print(dp[N]*2/(N*(N+1)))