#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <sstream>
#include<set>
#include<stack>

//#include<bits/stdc++.h>


using namespace std;	



int main() {

	long long int a, b;
	cin >> a >> b;

	for (long long int i = 1; i*i <= a + b; i++) {
		if ((a + b) % i)continue;
		if (i != a&&i != b && (a + i) % b == 0 && (b + i) % a == 0) {
			cout << i << endl;
			return 0;
		}
		long long int t = (a + b) / i;
		if (t != a&&t != b && (a + t) % b == 0 && (b + t) % a == 0) {
			cout << t << endl;
			return 0;
		}
	}
	cout << -1 << endl;
	



	return 0;




}