#include <bits/stdc++.h>

int main()
{
	long long A, B;
	scanf("%lld%lld", &A, &B);
	long long sum{A + B}, min{-1};
	for (long long i{1}; i * i <= sum; i++)
	{
		if (sum % i) continue;
		if ((A + i) % B == 0 && (B + i) % A == 0 && i != A && i != B)
		{
			min = i;
			break;
		}
		else if ((A + sum / i) % B == 0 && (B + sum / i) % A == 0 && sum / i != A && sum / i != B)
			min = sum / i;
	}
	printf("%lld\n", min);

	return 0;
}