#include <iostream.h>
#include <fstream.h>

//using namespace std;

 long factorial(long zero, long &x,long n) {
	//long x=j;
	//for (long i=n-1 ; i<=n ; i++) {
		x=x*n;
		while (x%10==0 && x/10!=0) {
			zero++;
			x=x/10;
		}
		x=x%10;
	//}
	return zero;
}

int main() {
	ifstream f("fact.in");
	ofstream g("fact.out");
	long p,n=1,zero=0;
	f>>p;
	f.close();
	long x=1;
	while (zero<p) {
		n++;
		zero=factorial(zero,x,n);
	}
	if (zero>p) g<<"-1";
	else g<<n;
	g.close();
	return 0;
}