#include <bits/stdc++.h>
using namespace std;

int main()
{
	auto check = [](int64_t a, int64_t b, int64_t c) {
		return ((a + b) % c == 0) && ((a + c) % b == 0) && ((b + c) % a == 0);
	};
	int64_t a, b;
	cin >> a >> b;
	int64_t ans = (1LL << 60);
	for (int64_t c = 1; c * c <= (a + b); c++)
	{
		if ((c == a) || (c == b))
		{
			continue;
		}
		if ((a + b) % c == 0)
		{
			if (check(a, b, c))
			{
				ans = min(ans, c);
			}
			if (check(a, b, (a + b) / c))
			{
				ans = min(ans, (a + b) / c);
			}
		}
	}
	cout << ((ans == (1LL << 60)) ? -1 : ans) << endl;
	return 0;
}