s = input()
l = len(s)
C = [0] * l #１~l文字までの文字の種類の数
dic ={}
k = 0
for i in range(l):
    if s[i] not in dic:
        dic[s[i]] = 1
        k += 1
    C[i] = k
w = sum(C)
sum_w = w

for i in range(1, l): #i番目から始める部分文字列
    b = s[i - 1] #手前の文字
    n = l - i
    if b in s[i:]:
        n = s[i:].index(b)
    w = w - n - 1
    sum_w += w

n = l * (l + 1) / 2
print(sum_w / n)