#include<iostream>
using namespace std;
int main (void){
long long n,k=2;
cin >> n;
long long count=0;
for(int i=1;i<=25;i++){
	k *=5;
	count += n/k;
}
  if(n%2==1){cout <<0;
  	}else{
		cout << count ;
  }
return 0;
}
