import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int K = scan.nextInt();
		scan.close();
		if(K == 0 || K > N) {
			System.out.println(0);
			System.exit(0);
		}
		if(N % 2 == 1 && K == N / 2 + 1) {
			System.out.println(N - 1);
			System.exit(0);
		}
		System.out.println(N - 2);
	}
}