def main():
    s = input()
    a = 0
    for i in range(1,len(s) // 2 + 1):
        a += 2 * i * (len(s) - i + 1)
    if len(s) % 2 != 0:
        a += (len(s) // 2 + 1) * (len(s) // 2 + 1)
    b = len(s) * (len(s) + 1) / 2
    print(a / b)
main()