#include <fstream.h>
long int n,i,j,nr,p;
ifstream f("fact.in");
ofstream g("fact.out");
int main()
{
	f>>n;
	f.close();
	nr=0;i=0;
	while(n>nr)
	{
		i=i+5;
		nr=nr+i/5;
	}
	g<<nr;
	g.close();
	return 0;
}