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