import java.util.Scanner;

/**
 * Main
 */
public class Main {

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

        double N = sc.nextInt();
        double D = sc.nextInt();

        double watch = 1 + D*2;

        double answer = Math.ceil(N/watch);

        // System.out.println(N);
        // System.out.println(watch);
        System.out.println((int)answer);


    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz