#include<stdio.h>
using namespace std;
int p,n,i,d,nr2,nr5,min1,p1;
int main()
{
	freopen("fact.in","r",stdin);
	freopen("fact.out","w",stdout);
	scanf("%d",&p);
	min1=-3;
	if(p<0)
	{
		printf("%d",-1);
		return 0;
	}
	else
	{
		while(p!=min1)
		{
			n=n+1;
			d=2;p1=d;nr2=0;
			while(p1<=n)
			{
				nr2=nr2+n/p1;
				p1=p1*d;
			}
			d=5;p1=d;nr5=0;
			while(p1<=n)
			{
				nr5=nr5+n/p1;
				p1=p1*d;
			}
			if(nr2>nr5)
			{
				min1=nr5;
			}
			else
			{
				min1=nr2;
			}
		}
	}
	printf("%d",n);
	return 0;
}