#include <bits/stdc++.h>
using namespace std;
long long int k, g, a, b, aa, bb;
int main() {
  cin >> k;
  a = 1;
  b = (long long int)sqrt((double)(4.0 * k * k - a * a) / 3.0);
  b -= (b % 2 == 0);
  g += (b - 1) / 2;
  aa = 1 + b / 2 * 3;
  bb = 1;
  while (a < aa) {
    a += 3;
    b++;
    while (a * a + 3 * b * b > 4 * k * k) b -= 2;
    bb++;
    g += (b - bb) / 2;
  }
  cout << 6 * g + 1;
  return 0;
}
