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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <algorithm>
#include <cmath>
#include <deque>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <map>
#include <numeric>
#include <queue>
#include <sstream>
#include <string>
#include <string.h>
#include <tuple>
#include <vector>

#define REP(i,x) for(int i{ 0 }; i < (int)(x); i++)
#define REPC(i,x) for(int i{ 0 }; i <= (int)(x); i++)
#define RREP(i,x) for(int i{ (int)(x) - 1 }; i >= 0 ;i--)
#define RREPC(i,x) for(int i{ (int)(x)}; i >= 0; i--)
#define REP1O(i,x) for(int i{ 1 }; i < (int)(x); i++)
#define REP1C(i,x) for(int i{ 1 }; i <= (int)(x); i++)
#define REPIT(i,x) for(auto i{(x).begin()}; i != (x).end(); i++) 

#define PB push_back
#define MP make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(),(x).end()

using namespace std;

typedef int64_t ll;
typedef double dbl;
typedef vector<bool> Vb;
typedef vector<char> Vc;
typedef vector<double> Vd;
typedef vector<int> Vi;
typedef vector<ll> Vl;
typedef vector<string> Vs;
typedef vector<vector<int>> VVi;
typedef vector<vector<ll>> VVl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ll, int> pli;

template<class T> inline bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
inline void swap(ll& a, ll& b) { a ^= b; b ^= a; a ^= b; }
inline void swap(int& a, int& b) { a ^= b; b ^= a; a ^= b; }
inline void YES() { cout << "YES" << endl; } inline void Yes() { cout << "Yes" << endl; }
inline void NO() { cout << "NO" << endl; } inline void No() { cout << "No" << endl; }

const int inf = 1 << 30;
const ll linf = 1LL << 60;
const int MOD = 1000000007;


int main()
{
	int n, d; cin >> n >> d;

	cout << (n + 2 * d) / (2 * d + 1) << endl;
}