#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-7;
bool judge(int x, int y, int k) {
  return 1LL * x * x + 3LL * y * y <= 4LL * k * k;
}
long long getAns(int k) {
  double sq3 = sqrt(3);
  int x, y;
  long long ans = 0;
  int cnt = 0;
  for (y = 1; true; y += 2) {
    if (judge(1, y, k))
      cnt++;
    else {
      y -= 2;
      break;
    }
  }
  cnt = cnt * 2 - 1;
  ans += cnt;
  x = 1;
  while (true) {
    y += 1;
    x += 3;
    cnt++;
    while ((!judge(x, y, k) || !judge(x + 1, y - 1, k)) && cnt > 0)
      y -= 2, cnt -= 2;
    if (cnt > 0)
      ans += cnt * 2;
    else
      break;
  }
  return ans;
}
int main() {
  int k;
  while (cin >> k) cout << getAns(k) << endl;
  return 0;
}
