s = input()
l = len(s)
C = [0] * l #１~l文字までの文字の種類の数
dic ={}
dic_a = {}
k = 0
for i in range(l):
    c = s[i]
    if c in dic:
        dic[c][0] += 1
    else:
        dic[c] = [1, [], 1]
        k += 1
    dic[c][1].append(i)
    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

    char_num, char_list, counter = dic[b]

    if counter < char_num:
        n = char_list[counter] - char_list[counter - 1] - 1
        dic[b][2] += 1

    w = w - n - 1
    sum_w += w

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