#include <bits/stdc++.h>
using namespace std;
int main() {
  double r, offset, n1, n2, r2, a, b;
  long long sum = 0, num;
  bool sig;
  cin >> r;
  r2 = r * r;
  for (b = 1, sig = true; b <= r; b += 1.5) {
    a = b - 0.5;
    n1 = sqrt((r2 - a * a) * 4 / 3);
    n2 = sqrt((r2 - b * b) * 4 / 3);
    if (sig)
      n1 -= 1;
    else
      n2 += 1;
    n1 /= 2;
    n2 /= 2;
    num = min((long long)n1, (long long)n2);
    if (b == 1)
      sum = num * 2 + 1;
    else if (sig)
      sum += num * 4 + 2;
    else
      sum += num * 4;
    sig = !sig;
  }
  cout << sum << endl;
}
