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();
        System.out.println(fucntion(N,K));
    }
    static int fucntion(int N, int K){
        if(K==0||K>N)return 0;
        if(N%2==0){
            return N-2;
        }else{
            if((N+2)/2==K)return N-1;
            return N-2;
        }
    }
}