#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7, INF = 1e17;
const int iINF = 1e9;
int main() {
  int a, b, c, n, m, k, d;
  string s;
  cin >> n >> d;
  a = d * 2 + 1;
  cout << n / a + min(1, n % a) << endl;
}
