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

#define rep(i, a, b) for (int i = a; i < b; i++)
using ll = long long;

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