#include <bits/stdc++.h>
using namespace std;
int radio;
long long res = 0, son = 0, col;
double x = -.5, y = .5;
const double tres = 1.7320508075688772935274463415059;
double dist(double x1, double y1, double x2, double y2) {
  return sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
}
bool dentro(double uno, double dos) {
  bool sip = true;
  uno += 1;
  sip = sip & (dist(uno, dos * tres, 0, 0) <= radio);
  uno += .5;
  dos -= .5;
  sip = sip & (dist(uno, dos * tres, 0, 0) <= radio);
  return sip;
}
int main() {
  cout.precision(17);
  cin >> radio;
  while (dentro(x, y)) {
    ++res;
    ++y;
  }
  --y;
  col = res - 1;
  res += res - 1;
  for (int va = 1; y > 0; va++) {
    if (va % 2 == 1) {
      ++col;
    }
    x += 1.5;
    y += .5;
    while (!dentro(x, y) and y > 0) {
      --col;
      --y;
    }
    if (!dentro(x, y)) {
      res += son * 2;
      cout << res << "\n";
      return 0;
    } else {
      son += col * 2;
      if (va % 2 == 0) {
        ++son;
      }
    }
  }
  return 0;
}
