fn main() {
    let mut nm = String::new();
    std::io::stdin().read_line(&mut nm).ok();
    let nm: Vec<usize> = nm.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let n = nm[0];
    let m = nm[1];

    if n == m {
        println!("1");
    } else if n % 2 == 0 && m >= n/2 {
        println!("2");
    } else {
        println!("-1");
    }
}