def SameRange(A):
    N=len(A)
    pre_same=[-1]*N
    next_same=[N]*N
    last_pos = defaultdict(lambda: -1)
    for i, a in enumerate(A):
        j = last_pos[a]
        if j==-1:
            last_pos[a] = i
        else:
            next_same[j] = i
            pre_same[i] = j
            last_pos[a] = i
    return pre_same, next_same

def count_pair(i,l,r):
    """
    部分列 [l,r] の中で、i を含む部分列の個数
    """
    return (i-l+1)*(r-i+1)


#######################################################################################
def example():
    global input
    example = iter(
        """
7
1 1 3 3 3 2 2
        """
            .strip().split("\n"))
    input = lambda: next(example)

#######################################################################################
import sys
input = sys.stdin.readline
from collections import defaultdict

# example()

A=[]
for s in input().rstrip():
    A.append(ord(s)-ord("a"))
N=len(A)
cnt=0
L,R = SameRange(A)
for i in range(N):
    l,r=L[i],R[i]
    cnt+=count_pair(i,l+1,N-1)

T=N*(N+1)//2
print(cnt/T)