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
	ruuu0048
*/
#include <stdio.h>
#include <math.h>
 
int main(void){
 
    int tree_num,view_scope; //リンゴの木の本数Nと監視員の視野の広さDの変数を宣言
    int watchman_num; //必要な監視員の数の変数を宣言
    float view_capacity; //一人の監視員が見られる木の本数

    scanf("%d",&tree_num); //Nを入力
    scanf("%d",&view_scope); //Dを入力

    view_capacity = view_scope * 2 + 1; //Dから一人の監視員が見られる本数を計算

    watchman_num = ceil(tree_num / view_capacity); //float型で計算したものを繰り上げ
 
    printf("%d\n",watchman_num); //必要な監視員の数をを出力
 
    return 0; 
 
}