#include <bits/stdc++.h>
using namespace std;
int main() {
  long long k;
  cin >> k;
  long long cnt = 0;
  for (double a = 2 + 2. / 3; a < (2 * k) / sqrt(3); a += 2) {
    double b =
        1e-8 + (-a + sqrt((16 * k * k) / 3. - (3 * a * a))) / 2. - 2. / 3;
    if (b > 0) cnt += ((int)b + 2) / 2;
  }
  cout << 1 + 6 * cnt << endl;
  return 0;
}
