#include <iostream>
#include<math.h>
using namespace std;

long long int a, b,c=-1;

int main(void) {
	cin >> a>> b;
	long long int i, k;
	for (k = 0; k * a - b <= a + b; k++) {
		if (a == b) break;
		if (k * a - b < 1) continue;
		i = k * a - b;
		if ((i + a) % b == 0 && (a + b) % i == 0 && a != i && b != i) {
			c = i;
			break;
		}
	}
	cout << c;
}