import java.util.Scanner;
class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();//14
	int d = sc.nextInt();//3
    int result = 0;
    
    if(n == 1 || n == 2){
      System.out.println(1);
    }else{
    for(int i = 0;i < n;i++){
      if((2*d+1)*i >= n){
        result = i;
        break;
      }
    }
    System.out.println(result);
    }
  }
}
/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