#include <stdio.h>

FILE* f;

int main()
{

	int p;
	int zecimale=-1;

	f=fopen ("fact.in","r");
	fscanf (f,"%d",&p);
	fclose (f);

	int i=1;
	int j;
	while (zecimale<p)
	{
		j=i;
		while (j%5==0)
		{
			zecimale++;
			j=j/5;
		}

		i++;
	}

	f=fopen ("fact.out","w");

	if (zecimale==-1)
		fprintf (f,"%d",zecimale);
	else
	{
		if (p!=0)
			fprintf (f,"%d",i-1);
		else
			fprintf (f,"%d",1);
	}

	fclose (f);

	return 0;
}