fn main() {
    let mut ab = String::new();
    std::io::stdin().read_line(&mut ab).ok();
    let mut ab = ab.trim().split_whitespace();
    let a: i32 = ab.next().unwrap().parse().unwrap();
    let b: i32 = ab.next().unwrap().parse().unwrap();

	let ans = match (a, b) {
		(_, 0) => 1,
		(-1, _) => 2,
		(_, _) => -1,
		};

	println!("{}", ans);
}