#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
	int n;
	while( cin >> n ){
		long long ans = 1;
		for(int i = 1; i <= n; i++) ans *= i;
		int cnt_zero = 0;
		while(1){
			if(ans % 10 == 0) cnt_zero++;
			else break;
		}
		cout << cnt_zero << endl;
	}
}