import java.util.*;

class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        long n = in.nextLong();
        long k = in.nextLong();

        long ans = 0;
        if(!(k == 0 || k > n )) {
            long mid = (n+1) / 2;
            if(mid != k) ans = n - 2;
            else if(mid == k) {
                if(n % 2 == 0) ans = n - 2;
                else ans = n - 1;
            }
        }

        System.out.println(ans);
    }
}