import java.util.*;

class Main {
   public static void main(String[] args) {
       Scanner sc = new Scanner(System.in);
       long N = sc.nextLong();
       long D = sc.nextLong();
       
       long m = N/(2*D+1);
       if(N%(2*D+1)!=0) {
       	m=m+1;
       }
       System.out.println(m);	
   }
}
       

/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