#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
const int M = 55;
const int K = 200010;
const int LIT = 2500;
const int INF = 1 << 30;
const int dir[6][2] = {{2, 0}, {-2, 0}, {1, 1}, {-1, 1}, {1, -1}, {-1, -1}};
bool check(long long x, long long y, long long n) {
  long long dist = x * x + 3 * y * y;
  long long cmp = 4 * n * n;
  return dist <= cmp;
}
bool inside(long long x, long long y, long long n) {
  for (int j = 0; j < 6; j++) {
    long long xx = x + dir[j][0];
    long long yy = y + dir[j][1];
    if (!check(xx, yy, n)) return 0;
  }
  return 1;
}
void solve(int n) {
  long long x = 0, y = 0;
  long long res = 1;
  while (1) {
    y += 2;
    if (!inside(x, y, n)) break;
    res += 2;
  }
  while (y >= 0) {
    x += 3, y++;
    while (y >= 0) {
      if (inside(x, y, n)) {
        res += (y + 1) * 2;
        break;
      }
      y -= 2;
    }
  }
  cout << res << endl;
}
int main() {
  int n;
  while (cin >> n) {
    solve(n);
  }
}
