import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		if (n <= m) {
		    System.out.println(1);
		} else {
		    if (n % 2 == 1 || n / 2 > m) {
		        System.out.println(-1);
		    } else {
		        System.out.println(2);
		    }
		}
   }
}