#include <fstream>

using namespace std;

ifstream intrare("factorial.in");
ofstream iesire("factorial.out");

int main()
{
	long int p,d;
	intrare>>p;
	int min= -1;
	long int a=1,b=100,c,e=0;
        while(a<=b)
	{
		c=a+(b-a)/2;
		d=c; e=0;
		while(d!=0)
		{
			d/=5;
			e+=d;
		}
		if(  p < e ) b=c+1;
		else if( p > e) a=c-1;
		else if(e==p) { min = c, b = c-1;}
	}
	iesire<<min;
	return 0;
}