import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int D = Integer.parseInt(sc.next());

        int p = 1 + D;
        int count = 0;
        if ( p < N ) {
            count++;
        } else {
            System.out.println(1);
            System.exit(0);
        }
        
        while (p < (N-D)) {
            p += (2*D+1);
            count++;
        }
            
        System.out.println(count);

    }
}
/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