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