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 Go (1.6) (judge verdict: Accepted)

package main
 
import (
	"bufio"
	"fmt"
	"log"
	"os"
	"strconv"
)
 
func run() {

	var N int64;
    N = nextInt();
    var D int64;
    D = nextInt();
	solve(N, D);

}

func solve(N int64, D int64){
	Println((N+(D*2)) / (D*2+1))
}


// --- template ---
// Thanks https://gist.github.com/halllllll/853ab587fd82ee3ffe6f7fb7fb499695
 
var sc = bufio.NewScanner(os.Stdin)
var out = bufio.NewWriter(os.Stdout)
 
// Caution: You will get unexpected result if a single token is larger than this size.
const BufferSize = 64 * 1024 * 1024
 
func Print(a ...interface{}) (n int, err error) {
	return fmt.Fprint(out, a...)
}
 
func Printf(format string, a ...interface{}) (n int, err error) {
	return fmt.Fprintf(out, format, a...)
}
 
func Println(a ...interface{}) (n int, err error) {
	return fmt.Fprintln(out, a...)
}
 
func next() string {
	sc.Scan()
	return sc.Text()
}
 
func nextInt() int64 {
	a, err := strconv.ParseInt(next(),10, 64)
	if err != nil {
		log.Fatal(err)
	}
	return a
}

func nextFloat() float64 {
	a, err := strconv.ParseFloat(next(), 64)
	if err != nil {
		log.Fatal(err)
	}
	return a
}


type IntHeap []int
func (h IntHeap) Len() int           { return len(h) }
func (h IntHeap) Less(i, j int) bool { return h[i] > h[j] }
func (h IntHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }

func (h *IntHeap) Push(x interface{}) {
	// Push and Pop use pointer receivers because they modify the slice's length,
	// not just its contents.
	*h = append(*h, x.(int))
}

func (h *IntHeap) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}

/////数字と文字
// 基本クラス
type NS struct {
    Num int
    Str string
}

//////　数字と数字
// 基本クラス
type NN struct {
    Num1 int
    Num2 int
}

 
func main() {
	sc.Buffer(make([]byte, 0, BufferSize), BufferSize)
	sc.Split(bufio.ScanWords)
	run()
	out.Flush()
}