import java.util.Scanner;

public class CheeseAndMouse{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int N = sc.nextInt();
        int K = sc.nextInt();
        if(K == 0 || K > N){
            System.out.println(0);
        }else if(N%2 == 1 && K == N/2+1){
            System.out.println(N-1);
        }else{
            System.out.println(N-2);
        }
    }
}