import java.io.*;
import java.util.Scanner;


public class Main_yukicoder816 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int a = sc.nextInt();
		int b = sc.nextInt();
		
		int ab = a + b;

		for (long c = 1; c * c <= ab; c++) {
			if (ab % c == 0) {
				if (check(a, b, c)) {
					pr.println(c);
					return;
				}
				if (c != ab / c) {
					if (check(a, b, ab / c)) {
						pr.println(ab / c);
						return;
					}
				}
			}
		}
		
		pr.println(-1);
	}

	private static boolean check(int a, int b, long c) {
		if (a == b || b == c || c == a) {
			return false;
		}
		
		if ((a + b) % c == 0 && (b + c) % a == 0 && (c + a) % b == 0) {
			return true;
		} else {
			return false;
		}
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(INPUT == null ? System.in : new ByteArrayInputStream(INPUT.getBytes()));
		pr = new Printer(System.out);

		solve();

//		pr.close();
		pr.flush();
//		sc.close();
	}

	static String INPUT = null;

	private static class Printer extends PrintWriter {
		Printer(OutputStream out) {
			super(out);
		}
	}
}