#include <bits/stdc++.h>
using namespace std;
long long int N;
long long int res() {
  long long int x = 1, y = 1, ans = 0;
  while (x * x + y * y * 3 <= N * N * 4) y += 2;
  ans += y - 2, x += 3, y--;
  while (y >= 0) {
    if (x * x + y * y * 3 > N * N * 4 ||
        (x + 1) * (x + 1) + (y - 1) * (y - 1) * 3 > N * N * 4)
      y -= 2;
    else
      ans += y * 2, x += 3, y++;
  }
  return ans;
}
int main() {
  scanf("%I64d", &N);
  printf("%I64d", res());
  return 0;
}
