fn main() {
    let mut xx = String::new();
    std::io::stdin().read_line(&mut xx).ok();
    let xx: Vec<i64> = xx.split_whitespace().flat_map(str::parse).collect();

    let n = match (xx[0], xx[1]) {
        (_, 0) => 1,
        (-1, _) => 2,
        _ => -1,
    };

    println!("{n}");
}