import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		sc.close();

		int max = (int) Math.sqrt(a + b);
		for (int i = 1; i <= max; i++) {
			if (a == i || b == i) {
				continue;
			}
			if ((a + b) % i == 0 && (a + i) % b == 0 && (i + b) % a == 0) {
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
	}
}