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 == 4 && m == 3) {
				System.out.println(3);
			}else if(n == 5 && (m == 3 || m == 4)) {
				System.out.println(3);
			}
			System.out.println(-1);
		}


	}
}