#include <bits/stdc++.h>
using namespace std;
double sqr(double x) { return x * x; }
int main() {
  double R, p, k, aux;
  long long ans = 0LL, t, a, b, mn;
  cin >> R;
  p = 1.5;
  for (int i = 0;; i++, p += 1.5) {
    if (i % 2) {
      if (sqr(R) - sqr(p + 0.5) < 0.0)
        a = 0LL;
      else {
        k = sqrt((4.0 * (sqr(R) - sqr(p + 0.5))) / 3.0);
        a = (long long)floor(k);
        a = (a + 1LL) / 2LL;
      }
      b = 0LL;
      if (sqr(R) - sqr(p + 1.0) < 0.0)
        b = 0LL;
      else {
        k = sqrt((sqr(R) - sqr(p + 1.0)) / 3.0);
        b = (long long)(floor(k));
      }
      if (!(p + 1.0 > R)) b++;
      mn = min(a, b);
      t = 2LL * mn - 1;
      if (t < 0LL) t = 0LL;
    } else {
      if (sqr(R) - sqr(p + 0.5) < 0.0)
        a = 0LL;
      else {
        k = sqrt((sqr(R) - sqr(p + 0.5)) / 3.0);
        a = (long long)(floor(k));
      }
      if (sqr(R) - sqr(p + 1.0) < 0.0)
        b = 0LL;
      else {
        k = sqrt((4.0 * (sqr(R) - sqr(p + 1.0))) / 3.0);
        b = (long long)(floor(k));
        b = (b + 1LL) / 2LL;
      }
      mn = min(a, b);
      t = 2LL * mn;
    }
    if (t == 0LL) break;
    ans += t;
  }
  ans *= 2LL;
  p = 0.0;
  if (sqr(R) - sqr(p + 0.5) < 0.0)
    a = 0LL;
  else {
    k = sqrt((4.0 * (sqr(R) - sqr(p + 0.5))) / 3.0);
    a = (long long)floor(k);
    a = (a + 1LL) / 2LL;
  }
  b = 0LL;
  if (sqr(R) - sqr(p + 1.0) < 0.0)
    b = 0LL;
  else {
    k = sqrt((sqr(R) - sqr(p + 1.0)) / 3.0);
    b = (long long)(floor(k));
  }
  if (!(p + 1.0 > R)) b++;
  mn = min(a, b);
  t = 2LL * mn - 1;
  if (t < 0LL) t = 0LL;
  ans += t;
  cout << ans << endl;
  return 0;
}
