import java.util.Scanner;

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

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


	}
}