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