import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        int min = Integer.MAX_VALUE;
        for (int i = 1; i <= Math.sqrt(sum); i++) {
            if (sum % i != 0) {
                continue;
            }
            if (i == a || i == b) {
                continue;
            }
            if (canSet(a, b, i)) {
                min = i;
                break;
            }
            if (sum / i == a || sum / i == b) {
                continue;
            }
            if (canSet(a, b, sum / i)) {
                min = Math.min(min, sum / i);
            }
        }
        if (min == Integer.MAX_VALUE) {
            System.out.println(-1); 
        } else {
            System.out.println(min);
        }
    }
    
    static boolean canSet(int a, int b, int c) {
        return (a + c) % b == 0 && (b + c) % a == 0;
    }
}