fn main() {
	let mut s = String::new();
	std::io::stdin().read_line(&mut s).ok();
	let n: Vec<u32> = s.split_whitespace().flat_map(str::parse).collect();
	println!(
		"{}",
		match n {
			_ if n[0] <= n[1] => 1,
			_ if n[0] % 2 == 0 && n[1] >= n[0] / 2 => 2,
			_ => -1,
		}
	)
}