MOD = 10 ** 9 + 7
INF = 10 ** 12
import sys
sys.setrecursionlimit(1000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)
from  itertools import permutations
from math import gcd

def main():
    s = input()
    n = len(s)
    dp = 0
    dic = {}
    ans = 0

    for i in range(1,n + 1):
        if s[i - 1] in dic:
            dp += i - dic[s[i - 1]]
            dic[s[i - 1]] = i
        else:
            dic[s[i - 1]] = i
            dp += i
        ans += dp
    
    ans /= (n + 1)*n//2
    print(ans)
if __name__ =='__main__':
    main()