import java.util.*;
public class Main{
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    float n = sc.nextFloat();
    float d = sc.nextFloat();
    float nd = n/((2*d)+1);
    int nd2 = (int)n/((2*(int)d)+1);
    if(n%((2*d)+1)>0){
      System.out.println(nd2+1);
    }else{
      System.out.println(nd2);
    } 
  }
}
/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