#include "bits/stdc++.h"

using namespace std;

void solve()
{
	long a, b;
	cin >> a >> b;
	if (a < b) swap(a, b);
	for (long i = 1; i * i <= a + b; i++)
	{
		if ((a + b) % i) continue;
		long c = (a + b) / i;
		if ((a + c) % b == 0 && (a + b) % c == 0 && (b + c) % a == 0)
		{
			cout << c << endl;
			return;
		}
	}
	cout << -1 << endl;
}

int main()
{
	solve();
	//cout << "yui(*-v・)yui" << endl;
	return 0;
}