Problem:
Score : 500 points

Problem Statement

For an integer n not less than 0, let us define f(n) as follows:

f(n) = 1 (if n < 2)

f(n) = n f(n-2) (if n \geq 2)

Given is an integer N. Find the number of trailing zeros in the decimal notation of f(N).

Constraints

0 \leq N \leq 10^{18}

Input

Input is given from Standard Input in the following format:

N

Output

Print the number of trailing zeros in the decimal notation of f(N).

Sample Input 1

12

Sample Output 1

1

f(12) = 12 × 10 × 8 × 6 × 4 × 2 = 46080, which has one trailing zero.

Sample Input 2

5

Sample Output 2

0

f(5) = 5 × 3 × 1 = 15, which has no trailing zeros.

Sample Input 3

1000000000000000000

Sample Output 3

124999999999999995

Solution in PyPy3 (2.4.0) (judge verdict: Wrong Answer)

import math


def main():
    N = int(input())

    # f(n) = 1 (n < 2)
    # f(n) = n * f(n - 2)
    # f(n) = n * (n - 2) * (n - 4) * ... * 4 * 2

    if (N % 2 == 0):
        c = 0
        a = 5
        while a <= N:
            c += math.floor(N / (2 * a))
            a *= 5
        print(c)

    else:
        print(0)


main()