package m.shin;
import java.util.Scanner;
public class Con531 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt() , m = s.nextInt();
		s.close();

		if(n == m){
			System.out.println(1);
		}else if(n % 2 == 0 && m >= n / 2){
			System.out.println(2);
		}else{
			System.out.println(-1);
		}
	}

}