# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	S = input()
	N = len(S)
	pre = [-1] * 26
	minus = 0
	for i in range(N):
		a = ord(S[i]) - 97
		num = i - pre[a] - 1
		minus += int(num * (num + 1) // 2)
		pre[a] = i

	for i in range(26):
		minus += int((N - 1 - pre[i]) * (N - pre[i]) // 2)

	# print(minus)

	ans = 26 - minus / (int(N * (N + 1) // 2))
	print(ans)



if __name__ == '__main__':
	main()