A = "abcdefghijklmnopqrstuvwxyz"
S = input()
l = len(S)

def f(n):
    return (n*(n+1))//2

ans = 26*f(l)

for e in A:
    t = 0
    for k in range(l):
        if S[k] == e:
            ans -= f(k-t)
            t = k+1
    ans -= f(l-t)

print(ans/f(l))