S = input()
N = len(S)
alp = 'abcdefghijklmnopqrstuvwxyz'
ans = 0
allp = N*(N+1)//2
for i in range(26):
    rm = 0
    cnt = 0
    for j in range(N):
        if S[j] == alp[i]:
            rm += cnt*(cnt+1)//2
            cnt = 0
        else:
            cnt += 1
    rm += cnt*(cnt+1)//2
    ans += (allp-rm)/allp
print(ans)