import java.util.*;
import java.math.*;

class main
{
	public static BigInteger trans(int x)
	{
		return BigInteger.valueOf(x);
	}
	public static void main(String[] args)
	{
		int []ans={-1,-1,-1,-1,3,-1,5,-1,7,7,7,-1,11,-1,13,7,7,-1,8,-1,19,19,7,-1,23,23};
		Scanner sc = new Scanner(System.in);
		BigInteger n = sc.nextBigInteger();

		if(n.compareTo( trans(25) )<=0)
			System.out.println(ans[n.intValue()]);
		else
		{
			if( n.mod(trans(8)).compareTo(trans(1))==0 && n.subtract(trans(8)).isProbablePrime(20) )
				System.out.println(14);
			else
				System.out.println(8);
		}
	}
}