// yukicoder: No.816 Beautiful tuples
// 2019.6.16 bal4u

#include <stdio.h>
#include <math.h>

int gcd(int a, int b) {
	int r;
	while (b != 0) r = a % b, a = b, b = r;
	return a;
}

int q[100], sz;
int A, B, C;

int check(int x) {
	if (x == A) return 0;
	if (x == B) return 0;
	if ((A+x) % B) return 0;
	return (B+x) % A == 0;
}

int main()
{
	int g, S, s;

	scanf("%d%d", &A, &B);
	g = gcd(A, B);
	if (g > 1) A /= g, B /= g;
	S = A + B, s = (int)sqrt((double)S);
	for (C = 1; C <= s; C++) {
		if (S % C == 0) {
			if (check(C)) break;
			q[sz++] = S/C;
		}
	}
	if (C <= s) C *= g;
	else {
		for (s = sz-1; ; s--) {
			if (s < 0) { C = -1; break; }
			if (check(q[s])) { C = g*q[s]; break; }
		}
	}
	printf("%d\n", C);
	return 0;
}