#include "bits/stdc++.h"
#define in std::cin
#define out std::cout
#define rep(i,N) for(LL i=0;i<N;++i)
typedef long long int LL;

std::vector<LL>divisor(LL num)
{
	std::vector<LL>res;
	for (LL i = 1; i * i <= num; ++i)
	{
		if (num % i == 0)
		{
			res.push_back(i);
			if ((num / i) != i) res.push_back(num / i);
		}
	}
	return res;
}

int main()
{
	LL A, B;
	in >> A >> B;

	auto divs = divisor(A + B);
	for (LL i : divs)
	{
		if (A != i && B != i && (A + i) % B == 0 && (B + i) % A == 0)
		{
			out << i << std::endl;
			return 0;
		}
	}
	out << -1 << std::endl;
}