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;
    	if ((a + 1) % b == 0 && (b + 1) % a == 0) {
    	    System.out.println(1);
    	    return;
    	}
    	TreeSet<Long> remain = new TreeSet<>();
    	remain.add(sum);
    	for (long i = 2; i <= Math.sqrt(sum); i++) {
    	    if (sum % i == 0) {
    	        if ((a + i) % b == 0 && (b + i) % a == 0) {
    	            System.out.println(i);
    	            return;
    	        }
    	        remain.add(sum / i);
    	    }
    	}
    	for (long x : remain) {
    	    if ((a + x) % b == 0 && (b + x) % a == 0) {
    	        System.out.println(x);
    	        return;
    	    }
    	}
    	System.out.println("-1");
	}
}