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