#include <string>
#include <iostream>

int main() {
	int a, b;
	std::cin >> a >> b;

	int c = 1;
	bool flg = false;
	while (c < a * b) {
		int sum1 = a + b;
		int sum2 = b + c;
		int sum3 = c + a;
		
		if (sum1 % c == 0 && sum2 % a == 0 && sum3 % b == 0) {
			std::cout << c << std::endl;
			return 0;
		}
		else {
			++c;
		}
	}
	c = -1;
	std::cout << c << std::endl;
}