S = input()
N = len(S)
M = N * (N + 1) // 2

ans = 0
for a in set(S):
    L = 0
    cnt = 0
    for i, s in enumerate(S):
        if a == s:
            size = i - L + 1
            cnt += size * (size - 1) // 2
            L = i + 1
    size = N - L + 1
    cnt += size * (size - 1) // 2
    ans += M - cnt

print(ans / M)