import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        long sum = a + b;
        long ans = -1;
        for (long i = 1; i <= Math.sqrt(sum); i++) {
            if (sum % i == 0) {
                if ((a + i) % b == 0 && (b + i) % a == 0 && a != i && b != i) {
                    ans = i;
                    break;
                }
                if ((a + sum / i) % b == 0 && (b + sum / i) % a == 0 && a != sum / i && b != sum / i) {
                    ans = sum / i;
                }
            }
        }
        System.out.println(ans);
    }
}