import java.util.*;

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