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 (7.3.0) (judge verdict: Wrong Answer)

import sys
read = sys.stdin.read
from math import log10, floor
def main():
    n = int(input())
    if n <= 9 or n % 2 == 1:
        print(0)
        sys.exit()
    tanni = 0
    keta = floor(log10(n))
    r = 0
    n2 = list(map(int, str(n)))[::-1]    
    for i1 in range(1, keta + 1):
        tanni = tanni * 10 + 1
        print(tanni)
        r += tanni * n2[i1]
    print(r)

if __name__ == '__main__':
    main()