en2asc = lambda s: ord(s) - 97

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

dp = [1] * L
for i in range(1, L):
    dp[i] = dp[i - 1] + i - left[i]
ans = sum(dp) * 2 / ((L + 1) * L)
print(ans)