#include<fstream.h>
int factorial(int N)
{
	int k=0,x=1,j;
	for(j=1;j<=N;j++)x=x*j;
	while(x%10==0&&x)
	{
		k++;x/=10;
	}
	return k;
	
}
int main()
{
	ifstream f("fact.in");
	ofstream g("fact.out");
	int P,N,ok=0;
	f>>P;
	for(N=1;N++;)
	{
		if(factorial(N)==P)
		{
			g<<N;ok=1;
			break;
		}
	}
	if(ok==0)g<<"-1";
	f.close();
	g.close();
	return 0;
}