*s, = map(ord,input())
n = len(s)
cnt = 26*n*(n+1)//2
res = [-1]*26
for i,si in enumerate(s):
    si -= 97
    cnt -= (i-res[si])*(i-res[si]-1)//2
    res[si] = i
for i in range(26):
    cnt -= (n-res[i])*(n-res[i]-1)//2
print(cnt*2/n/(n+1))