import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner s = new Scanner(System.in);
        String[] z =s.nextLine().split(" ");
        int n=Integer.parseInt(z[0]);
        int d=Integer.parseInt(z[1])*2+1;
        for(int i=1;i<= 1000;i++){
            if(n <= d*i){
                System.out.println(i);
                break;
            }
        }
    }
}
/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