S = input()
l = len(S)

dp = [[1 for i in range(l)] for j in range(l)]

sum_v = 0
for i in range(0, l):
    for j in range(i + 1, l):
        if S[j] in S[i:j - 1]:
            dp[i][j] = dp[i][j - 1]
        else:
            dp[i][j] = dp[i][j - 1] + 1
        sum_v += dp[i][j]

n = l * (l + 1) / 2
print((sum_v + l) / n)