import java.util.Scanner;

public class No531 {

	public static void main(String[] args) {
		No531 no531 = new No531();
		no531.mainStart();
	}

	public void mainStart() {
		Scanner sc = new Scanner(System.in);
		String[] nextLines = sc.nextLine().split(" ");
		int animalNumber = Integer.parseInt(nextLines[0]);
		int bootRideNumber = Integer.parseInt(nextLines[1]);
		int ans = logic(animalNumber, bootRideNumber);
		System.out.println(ans);
	}

	public int logic(int animalNumber, int bootRideNumber) {
		if (animalNumber==bootRideNumber) {
			return 1;
		}else if (animalNumber%2==0) {
			if (animalNumber <= bootRideNumber * 2) {
				return 2;
			}
		}else {
			return -1;
		}
		return -1;
	}

}