use proconio::input;

fn main() {
    input! {
        n: usize, m: usize,
    }

    let mut x = 0;
    for i in 0..30 {
        let n = n >> i & 1;
        let m = m >> i & 1;
        let Some(k) = (0..2).find(|&k| m == if i % 2 == 0 { n & k } else { n | k }) else {
            println!("-1");
            return;
        };
        x |= k << i;
    }
    println!("{x}");
}