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 <bits/stdc++.h>
using namespace std;

/*
#include <boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
using cint = cpp_int;
*/

// Define
using ll = long long;
using ull = unsigned long long;
using ld = long double;
const ll dx[4] = {1, 0, -1, 0};
const ll dy[4] = {0, 1, 0, -1};
const ll MOD = 1e9 + 7;
const ll mod = 998244353;
const ll inf = 1 << 30;
// const ll INF = LONG_MAX;
const ll INF = 1LL << 60;
const ull MAX = ULONG_MAX;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define elif else if
#define endl '\n'
#define space ' '
#define def inline auto
#define func inline constexpr ll
#define run(a) __attribute__((constructor)) def _##a()
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
#define input(a) scanf("%lld", &(a))
#define print(a) printf("%lld\n", (a))
#define fi first
#define se second
#define ok(a, b) (0 <= (a) && (a) < (b))
#define modulo(a) ((a % mod + mod) % mod)
template <class T> using vvector = vector<vector<T>>;
template <class T> using pvector = vector<pair<T, T>>;
template <class T>
using rpriority_queue = priority_queue<T, vector<T>, greater<T>>;

template <class T> bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> bool chmin(T &a, const T &b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}

// Debug
#define debug(...)                                                             \
    {                                                                          \
        cerr << __LINE__ << ": " << #__VA_ARGS__ << " = ";                     \
        for (auto &&X : {__VA_ARGS__}) cerr << "[" << X << "] ";               \
        cerr << endl;                                                          \
    }

#define dump(a, h, w)                                                          \
    {                                                                          \
        cerr << __LINE__ << ": " << #a << " = [" << endl;                      \
        rep(__i, h) {                                                          \
            rep(__j, w) cerr << a[__i][__j] << space;                          \
            cerr << endl;                                                      \
        }                                                                      \
        cerr << "]" << endl;                                                   \
    }

#define vdump(a, n)                                                            \
    {                                                                          \
        cerr << __LINE__ << ": " << #a << " = [";                              \
        rep(__i, n) if (__i) cerr << space << a[__i];                          \
        else cerr << a[__i];                                                   \
        cerr << "]" << endl;                                                   \
    }

struct edge {
    ll to, cost;
    edge(ll a, ll b) : to(a), cost(b) {}
};

struct position {
    ll x, y;
    position() {}
    position(ll a, ll b) : x(a), y(b) {}
    position next(ll i) { return {x + dx[i], y + dy[i]}; }
    ll mdist() { return abs(x) + abs(y); }
    double dist() { return sqrt(x * x + y * y); }
    double norm(ll d) {
        if (d == inf) return max(x, y);
        if (d == 1) return mdist();
        if (d == 2) return dist();
        return 0;
    }
    ll num(ll width) { return abs(x) * width + abs(y); }

    bool operator==(position a) { return x == a.x && y == a.y; }
    bool operator!=(position a) { return x != a.x || y != a.y; }
    bool operator<(position a) { return x < a.x && y < a.y; }
    bool operator>(position a) { return x > a.x && y > a.y; }
    bool operator<=(position a) { return x <= a.x && y <= a.y; }
    bool operator>=(position a) { return x >= a.x && y >= a.y; }
    position operator+(position a) { return position(x + a.x, y + a.y); }
    position operator-(position a) { return position(x - a.x, y - a.y); }
    position operator*(position a) { return position(x * a.x, y * a.y); }
    position operator/(position a) { return position(x / a.x, y / a.y); }
    position operator%(position a) { return position(x % a.x, y % a.y); }
    position complex(position a) {
        return position(x * a.x - y * a.y, x * a.y + y * a.x);
    }
    /*
        // for sort:
        bool operator<(position a) { return x ^ a.x ? x < a.x : y < a.y; }
        bool operator>(position a) { return x ^ a.x ? x > a.x : y > a.y; }
        bool operator<=(position a) { return x ^ a.x ? x < a.x : y <= a.y; }
        bool operator>=(position a) { return x ^ a.x ? x > a.x : y >= a.y; }
    */
};
position Origin = position(0, 0);
using pos = position;
using vec = position;

struct Range {
    ll left, right;
    Range() {}
    Range(ll l, ll r) : left(l), right(r) {}
    ll length() { return right - left; }
    bool operator==(Range A) { return left == A.left && right == A.right; }
    bool operator!=(Range A) { return !(Range(left, right) == A); }
    bool operator>(Range A) { return left < A.left && right > A.right; }
    bool operator<(Range A) { return left > A.left && right < A.right; }
    bool operator>=(Range A) { return left <= A.left && right >= A.right; }
    bool operator<=(Range A) { return left >= A.left && right <= A.right; }
};

// Loop
#define inc(i, a, n) for (ll i = (a), _##i = (n); i <= _##i; ++i)
#define dec(i, a, n) for (ll i = (a), _##i = (n); i >= _##i; --i)
#define rep(i, n) for (ll i = 0, _##i = (n); i < _##i; ++i)
#define each(i, a) for (auto &&i : a)
#define loop() for (;;)

// Stream
#define fout(n) cout << fixed << setprecision(n)
#define fasten cin.tie(0), ios::sync_with_stdio(0)

// Speed
run(0) { fasten, fout(10); }
#pragma GCC optimize("O3")
#pragma GCC target("avx")

// Gen-Test
#define RAND(a)                                                                \
    {                                                                          \
        random_device rnd;                                                     \
        mt19937_64 a(rnd());                                                   \
    }
#define warning(A)                                                             \
    if (!(A)) return 1
#define sin(a, b) ifstream a(b);
#define sout(a, b) ofstream a(b);

// Math
//#define gcd __gcd
func gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
func lcm(ll a, ll b) { return a * b / gcd(a, b); }
func sign(ll a) { return a ? abs(a) / a : 0; }

template <class T> def in() {
    T A;
    cin >> A;
    return A;
}

template <class T>
def out(vector<vector<T>> A, ll H, ll W, char divc = space, char endc = endl) {
    rep(i, H) {
        rep(j, W) {
            if (j)
                cout << divc << A[i][j];
            else
                cout << A[i][j];
        }
        cout << endc;
    }
}

signed main() {
    ll N, D;
    cin >> N >> D;
    D = D * 2 + 1;
    cout << (N + D - 1) / D << endl;
}

// 今年残り 325AC
// for compilation: g++ -Ofast -march=native -o _ _.cpp -std=c++20