#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Trim().Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

var n = I<int>();
var m = I<int>();

int Solve()
{
    var res = 0;
    for (var i = 0; i < 30; i++)
    {
        var d = 1 << i;
        var fn = (n & d) != 0;
        var fm = (m & d) != 0;
        if (i % 2 == 0)
        {
            if (fm)
            {
                if (!fn) return -1;
                res |= d;
            }
        }
        else
        {
            if (!fm)
            {
                if (fn) return -1;
            }
            else if (!fn) res |= d;
        }
    }
    return res;
}

Console.WriteLine(Solve());