#include<iostream>
using namespace std;
int main(){
	int n, cnt_zero;
	long long ans;
	while( cin >> n ){
		ans = 1.0;
		cnt_zero = 0;
		for( int i = 1; i <= n; i++ ) 
			ans *= i;
		while( ans % 10 == 0 ) 
			cnt_zero++;
		cout << cnt_zero << '?\n';
	}
}