import java.util.Scanner;

public class Main {
    static Scanner in = new Scanner(System.in);

    public static void main(String[] args) {
        int N = in.nextInt(), K = in.nextInt();
        if (K < 1 || N < K) {
            System.out.println(0);
        } else if (K == N / 2 + 1 && N % 2 == 1) {
            System.out.println(N - 1);
        } else {
            System.out.println(N - 2);
        }
    }
}