#include <string>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <list>
#include <stdexcept>
#include <functional>
#include <utility>
#include <iomanip>
#include <ctime>
#include <valarray>
#include <iostream>
#include <sstream>
#include <fstream>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
#define MEM(a,b) memset((a),(b),sizeof(a))
const LL INF = 1e9 + 7;
const int N = 2e5 + 10;

int main()
{
	//freopen("input.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);
	LL n, m;
	cin >> n >> m;
	LL ans = 0;
	for (int i = 0; i <= 30; i++)
	{
		int x = n >> i & 1;
		int y = m >> i & 1;
		int k = -1;
		for (int o = 0; o < 2; o++)
		{
			if (i % 2 == 0)
			{
				if ((o & x) == y)
				{
					k = o;
					break;
				}
			}
			else
			{
				if ((o | x) == y)
				{
					k = o;
					break;
				}
			}
		}
		if (k == -1) cout << -1 << endl, exit(0);
		ans |= k << i;
	}
	cout << ans << endl;
	return 0;
}