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