#include<fstream>
using namespace std;
int main()
{
	ifstream in("fact.in");
	ofstream out("fact.out");
	long n=0,s=0,p,i=0,r=1;
	in>>p;
	while(s!=p){
		n++;
		i++;
		r=r*i;
		while(r%10==0){
			r=r%10;
			s++;
		}
	}
	out<<n;
	in.close();
	out.close();
	return 0;
}