#include<iostream>
#include<string>
#include<vector>
using namespace std;

int Enjapma(int A, int B) {
	if (A == B || A<1 || B<1) {
		return -1;
	}

	int D = A + B;
	vector<int> div;
	for (int i = 1; i*i <= D ; i++) {
		if (D%i == 0) {
			div.push_back(i);
			if (i != D / i) { div.push_back(D / i); }
		}
	}
	
	int ans = -1;
	vector<int> Bans;
	vector<int>::iterator a = div.begin();
	for (int i = 0; i < div.size(); i++) {
		if ((A + *a) % B == 0 && (B + *a) % A == 0) {
			return *a;
		}
		a++;
	}

	return ans;

}
int main() {
	int A, B;
	cout << "A B > "; cin >> A >> B;
	
	cout << Enjapma(A, B) << endl;

	return 0;

}