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)

/* kadai4_1
   reo012 */

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

int main(void){
    // 途中で割り算をし、小数の切り上げをするので、
    // double型でn,dを取得。答えは整数で得られる。
    double n, d;
    int ans;
    scanf("%lf %lf", &n, &d);
    // 一人で最大（1+2*d）本の木を監視できるので、
    // それでnを割って小数切り上げる。
    ans = ceil(n/(1+2*d));
    printf("%d\n", ans);
    return 0;
}