#include <bits/stdc++.h>
using namespace std;

int main() {
	int n, m; cin >> n >> m;
	int ans = 0;
	for (int i=0; i<30; i++) {
		int x = (n >> i) & 1, y = (m >> i) & 1;
		int k;
		if (i % 2 == 0) {
			if (y == 0) k = 0;
			else if (x == 1) k = 1;
			else {
				cout << -1 << endl;
				return 0;
			} 
		} else {
			if (x == y) k = 0;
			else if (x == 0 && y == 1) k = 1;
			else {
				cout << -1 << endl;
				return 0;
			}
		}
		ans |= k << i;
	}
	cout << ans << endl;
}