fn main() {
    let mut xx = String::new();
    std::io::stdin().read_line(&mut xx).ok();
    let xx: Vec<u64> = xx.split_whitespace().flat_map(str::parse).collect();

    if xx[1] <= xx[0] {
        println!("{}", xx[1] - 1);
    } else {
        println!("-1");
    }
}