Problem:
Score : 200 points

Problem Statement

There are N apple trees in a row. People say that one of them will bear golden apples.

We want to deploy some number of inspectors so that each of these trees will be inspected.

Each inspector will be deployed under one of the trees. For convenience, we will assign numbers from 1 through N to the trees. An inspector deployed under the i-th tree (1 \leq i \leq N) will inspect the trees with numbers between i-D and i+D (inclusive).

Find the minimum number of inspectors that we need to deploy to achieve the objective.

Constraints

All values in input are integers.

1 \leq N \leq 20

1 \leq D \leq 20

Input

Input is given from Standard Input in the following format:

N D

Output

Print the minimum number of inspectors that we need to deploy to achieve the objective.

Sample Input 1

6 2

Sample Output 1

2

We can achieve the objective by, for example, placing an inspector under Tree 3 and Tree 4.

Sample Input 2

14 3

Sample Output 2

2

Sample Input 3

20 4

Sample Output 3

3

Solution in C (GCC 5.4.1) (judge verdict: Accepted)

/*  ex4-1
    kosaq       */

#include <stdio.h>
#include <math.h>

int main(void){
    double n_tree;//N本の林檎の木をn_treeで定義
    double d_range;//一人は、D本先までを監視できるのでDをd_rangeで定義
    double one_observe;//一人が監視できる林檎の木の数
    int people_number;//N本の林檎の木を監視するために必要な人数の最小値

    scanf("%lf %lf", &n_tree, &d_range);//NとDを入力

    one_observe = d_range * 2 + 1;//一人が監視できる林檎の木の数
    people_number = ceil(n_tree / one_observe);//N本の林檎の木を監視するために必要な人数の最小値は(n_tree / one_observ)eの小数値を切り上げた整数
    printf("%d\n", people_number);//N本の林檎の木を監視するために必要な人数の最小値を出力

    return 0;
}