#include "bits/stdc++.h"

using namespace std;

void solve()
{
	long a, b;
	cin >> a >> b;
	long c = abs(a - b);
	if ((a + c) % b || (a + b) % c)
	{
		cout << -1 << endl;
		return;
	}
	cout << c << endl;
}

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