#!/usr/bin/env python3
# %%
stdin = open(0)

# %%
import numpy as np


# %%
S = np.frombuffer(stdin.buffer.read().rstrip(), 'S1')


# %%
n = len(S)
total = n * (n + 1) // 2
numerator = 0
for i in range(26):
    alphabet = chr(ord('a') + i).encode()
    ind = np.hstack([[-1], np.where(S == alphabet)[0], [n]])
    gaps = ind[1:] - ind[:-1] - 1
    numerator += total - (gaps * (gaps + 1) // 2).sum()

# %%
answer = numerator / total
print(answer)