#include<bits/stdc++.h>
using namespace std;
int main(){
	long long N,resu,ol;
	cin>>N;
	if(N<2){
		resu=1;
	} else{
		if(N%2 !=0){
			resu=0;
		}else {
			resu=N/10;
			ol=resu;
			while(ol != 0){
			resu= resu+ol/5;
			ol= ol/5;	
			}
			
		}
	} 	
	cout<<resu;
	return 0;
}