en2asc = lambda s: ord(s) - 97

S = input()
L = len(S)
cur = [-1] * 26
dp = [0] * (L + 1)
for i, s in enumerate(S):
    x = en2asc(s)
    dp[i + 1] = dp[i] + i - cur[x]
    cur[x] = i

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