package m.shin;
import java.util.Scanner;
public class Con521 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int n = s.nextInt() , k = s.nextInt();
		s.close();

		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);
		}

	}

}