s = input().strip()
n = len(s)
from collections import defaultdict

positions = defaultdict(list)
for idx, char in enumerate(s):
    positions[char].append(idx)

total = 0
for ch in positions:
    prev = -1
    for i in positions[ch]:
        left = i - prev
        right = n - i
        total += left * right
        prev = i

num_substrings = n * (n + 1) // 2
average = total / num_substrings

# Print with 5 decimal places
print("{0:.5f}".format(average))