#include <bits/stdc++.h>
using namespace std;
const int oo = 1000000000;
const double H = sqrt(3.0) * 0.5;
const double eps = 1e-6;
int main() {
  double r;
  scanf("%lf", &r);
  int i;
  long long ans = 0;
  for (i = 0; i <= r; i++) {
    double x = 1.5 * i + 1.0;
    if (x > r) break;
    double y = sqrt(r * r - x * x);
    int k;
    if (i % 2)
      k = (floor(y / H + eps) + 1) / 2;
    else
      k = floor(y / H + eps) / 2 + 1;
    x = 1.5 * i + 0.5;
    y = sqrt(r * r - x * x);
    if (i % 2)
      k = 2 * min(k, (int)floor(y / H + eps) / 2);
    else
      k = 2 * min(k, ((int)floor(y / H + eps) + 1) / 2) - 1;
    ans += i ? 2 * k : k;
  }
  cout << ans << endl;
}
