S = input()
N= len(S)

dic = {}
dic[S[0]] = 0
dp = [0]*N
dp[0] = 1
mod = N*(N+1)//2
for i in range(1, N):
    if S[i] in dic:
        dp[i] = dp[i-1] + i - dic[S[i]]
    else:
        dp[i] = dp[i-1] + i + 1
    dic[S[i]] = i

print(sum(dp)/mod)