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 C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>

int function(int x);

int main()
{
  int N;
  int ans = 0;
  int s = 0;

  scanf("%d",&N);
 
  s = function(N);

  while(s%10 == 0){
    s = s/10;
    ans++;
  }

  printf("%d",ans);
   
  return 0;
}

int function(int x){
  if(x < 2){
    return 1;
  }
  else{
    return x*function(x - 2);
  }
}