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] == 0 || xx[1] > xx[0] {
        println!("0");
    } else if xx[0] & 1 == 1 && xx[1] == xx[0] / 2 + 1 {
        println!("{}", xx[0] - 1);
    } else {
        println!("{}", xx[0] - 2);
    }
}