#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main(){
	int A, B;
	cin >> A >> B;
	vector<int> f;
	for (int i = 1; i * i <= A + B; i++){
		if ((A + B) % i == 0){
			f.push_back(i);
			f.push_back((A + B) / i);
		}
	}
	sort(f.begin(), f.end());
	for (int C : f){
		if ((B + C) % A == 0 && (C + A) % B == 0 && C != A && C != B){
			cout << C << endl;
			return 0;
		}
	}
	cout << -1 << endl;
}