import java.util.Scanner;

public class N521 {

	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		int N=sc.nextInt();
		int K=sc.nextInt();
		int max=0;

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

	}

}