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

    if k == 0 || k > n {
        println!("0");
        return;
    }
    if n % 2 == 1 && (n+1) / 2 == k {
        println!("{}", n-1);
    } else {
        println!("{}", n-2);
    }
}