#include <stdio.h>

int main()
{
	int A, B;
	scanf("%d%d", &A, &B);

	if (A > B) {
		int _ = A;
		A = B;
		B = _;
	}

	int C = -1;
	if (2*A == B)
		C = A + B;
	else if (3*A == 2*B)
		C = B - A;
	else if (3*A == B)
		C = B - A;

	printf("%d\n", C);
}