import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Main main = new Main();
		main.solveA();
	}

	private void solveA() {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int K = sc.nextInt();
		if (N == K * 2 - 1) {
			System.out.println(N - 1);
		} else if (K == 0 || N < K) {
			System.out.println(0);
		} else {
			System.out.println(N - 2);
		}
	}
}