import java.util.*;

public class Main{
    public static void main(String args[] ) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        if (k == 0 || k > n) {
            System.out.println(0);
        } else if (n % 2 == 1 && k == (n + 1) / 2) {
            System.out.println(n - 1);
        } else {
            System.out.println(n - 2);
        }
    }    
}