s = input()
l = len(s)
m = [0] * l
m[-1] = 1
t = 1  # 直前の文字の種類数
for i in range(l - 2, -1, -1):
    if not s[i] in s[i + 1: l]:
        t += 1
    m[i] = t
x = 0
for i in range(l):
    x += m[i] * (i + 1)

print(2 * x / (l + l * l))