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
   Sagzar1993  */


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

int main(void) {
    int treeNum, halfRange;  //木の本数と、監視できる片側範囲

    scanf("%d %d", &treeNum, &halfRange); //入力を受け取る

    int range = halfRange * 2 + 1;  //１人の監視員が監視できる木の最大本数

    /* 木の本数を、１人が監視できる最大の木の本数で割る
       余った木にはもう１人の監視員が必要なので切り上げる
       treeNumはint型からdouble型に変換 */
    double observerNum = ceil((double) treeNum / range);

    printf("%d\n", (int) observerNum); //整数に直して出力

    return 0;
}