S = input()
res = 0
l = [-1]*26
for i in range(len(S)):
    p = l[ord(S[i])-97]
    if p!=-1:
        res += (i-p)*(len(S)-i)
    else:
        res += (i+1)*(len(S)-i)
    l[ord(S[i])-97] = i
print(2*res/((1+len(S))*len(S)))