using namespace std;
#include<fstream>
int main()
{
	long long P,i,nr=0;
	ifstream fcin("fact.in");
	ofstream fcout("fact.out");
	fcin>>P;
	if (P<0)
		fcout<<-1;
	else
		if(P==0)
			fcout<<1;
		else
		{
			long long a=P*5,i,asa=25;
			for(;asa<P*5;asa*=5)
				for (i=asa;i<=P*5;i*=asa)
					a-=5;
			fcout<<a;
		}
	fcin.close();
	fcout.close();
	return 0;
}