import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
    long ans = (long)Math.pow(10, 18) + 1;
    for(long i = 1; (i * i) <= (a + b); i++) {
      if(((a + b) % i) == 0) {
        long t = (a + b) / i;
        if((((b + i) % a) == 0) && (((a + i) % b) == 0)) {
          ans = Math.min(ans, i);
        }
        if((((b + t) % a) == 0) && (((a + t) % b) == 0)) {
          ans = Math.min(ans, t);
        }
      }
    }
    if(ans == ((long)Math.pow(10, 18) + 1)) ans = -1;
    System.out.println(ans);
  }
}