#include <fstream>
using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
int main()
{
	long int p=0,cinci=0,aux=0,plus=0;
	double putere=0;
	in>>p;
	cinci=5;
	while(putere/5<p)
	{
		aux=cinci;
		plus=0;
		while(aux%5==0)
		{
			++plus;
			aux=aux/5;
		}
		putere=putere+plus*5;
		cinci+=5;
	}
	if (putere/5>p)
		putere=putere-5*(p-5*putere);
	if(p==0) 
		out<<"1";
	else 
		out<<putere;
	return 0;
}