#include <iostream>
#include <fstream>

using namespace std;

unsigned long long factorial(long n) {
	long x=1,zero=0;
	for (long i=2 ; i<=n ; i++) {
		x=x*i;
		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");
	unsigned long long p,n=1,zero=0;
	f>>p;
	f.close();
	while (zero<p) {
		n++;
		zero=factorial(n);
	}
	if (zero>p) g<<"-1";
	else g<<n;
	g.close();
	return 0;
}