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 JavaScript (node.js v5.12) (judge verdict: Accepted)

// inputに入力データ全体が入る
function Main(input) {
	input = input.split(" "); // 1行目がinput[0], 2行目がinput[1], …に入る
    // ここに処理を書く
    var N = input[0]
    var D = input[1]
    var count = 0
    var range = 0
    while (range < N) {
        range = range + Number(D) + 1 + Number(D)
        count++
    }

    
	// console.logは標準出力に1行出力します (自動的に最後に改行が入ります)
	console.log(count);
}

// ※この行以降は編集しない (標準入力から一度に読み込みMainを呼び出します)
Main(require("fs").readFileSync("/dev/stdin", "utf8"));