import java.util.Scanner;
public class Main {
public static void main(String args[]) {
	Scanner sc = new Scanner(System.in);

	int N = sc.nextInt();

	int D = sc.nextInt();

	if(N%(2*D+1) !=0) {
		System.out.print(N/(2*D+1) +1);
	}

	else {System.out.print(N/(2*D+1));}


}
}
/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