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 Kotlin (1.0.0) (judge verdict: Runtime Error)

import java.io.PrintWriter
val pw = PrintWriter(System.out)
fun main(args : Array<String>) {
    func()
    pw.flush()
}

fun func() {
    val n = nextInt()
  	val d = nextInt()
  	val onePerson = 2*d+1
  	val ans:Int = n/onePerson
  	println(if(ans*onePerson<n) ans+1 else ans)
}

// 入力取得
fun next() = readLine()!!
fun nextInt() = next().toInt()
fun nextLong() = next().toLong()
fun nextDouble() = next().toDouble()
fun listOfString() = next().split(" ")
fun listOfInt() =listOfString().map { it.toInt() }
fun listOfLong() =listOfString().map { it.toLong() }
fun listOfDouble() =listOfString().map { it.toDouble() }

// 約数のList
fun divisor(value : Long) : List<Long> {
    val max = Math.sqrt(value.toDouble()).toLong()
    return (1..max)
        .filter { value % it == 0L }
        .map { listOf(it, value / it) }
        .flatten()
        .sorted()
}

// 範囲内の素数を取得
// fromだけ指定すると戻り値の個数で素数判定ができる
fun prime(from : Long, to : Long = from) : List<Long> {
    return (from..to).filter { i ->
        val max = Math.sqrt(i.toDouble()).toLong()
        (2..max).all { j ->  i % j != 0L }
    }
}

// 素因数分解
fun decom(value : Long) : List<Long>{
    if (value == 1L) return listOf(1)
    val max = Math.sqrt(value.toDouble()).toLong()
    return prime(2, max).filter { value % it == 0L }
}

// 最大公約数
fun gcd(a : Long, b : Long) : Long {
    return if (a % b == 0L) b else gcd(b, a % b)
}

// 文字列を入れ替え
fun swap(base : String, a : String, b : String) : String {
    return base.map {
        when (it) {
            a.toCharArray()[0] -> b
            b.toCharArray()[0] -> a
            else -> it.toString()
        }
    }.joinToString()
}

fun println(value : Any) {
    pw.println(value)
}