import java.util.*;


public class Main {
    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 == 0){
            System.out.println(N-2);
        }
        else{
            if( ( (N+1)/2) == K ){
                System.out.println(N-1);
            }
            else{
                System.out.println(N-2);
            }
        }
    }
}