#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-10;
const double H = 0.5 * sqrt(3.0);
int ifloor(double x) { return (int)floor(x); }
int main() {
  int n;
  cin >> n;
  double r = n;
  long long ans = 0;
  for (int i = 0; i <= n; i++) {
    double x, y;
    x = 1.5 * i + 1.0;
    if (x > r) break;
    y = sqrt(r * r - x * x);
    int k;
    if (i % 2)
      k = (ifloor(y / H) + 1) / 2;
    else
      k = ifloor(y / H) / 2 + 1;
    x = 1.5 * i + 0.5;
    y = sqrt(r * r - x * x);
    if (i % 2)
      k = 2 * min(k, ifloor(y / H) / 2);
    else
      k = 2 * min(k, (ifloor(y / H) + 1) / 2) - 1;
    ans += i ? 2 * k : k;
  }
  cout << ans << endl;
}
