#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const long double height = sqrt(3.0) / 2;
const long double width = 1.0;
bool ok(long double nx, long double ny, long double r) {
  long double x = nx * width;
  long double y = ny * height;
  x += width * 0.5;
  y += height;
  return x * x + y * y < r * r + 1e-10;
}
int main() {
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  long double x = 1.5, py = 1.0;
  while (ok(x, py, n)) py += 2.0;
  long long ans = 0;
  long double y = height;
  while (ok(x, y, n)) {
    while (!ok(x, py, n)) py -= 2.0;
    ans += (long long)((py - y) / (2.0) + 1.5);
    x += 1.5;
    y += 1.0;
    py += 1.0;
  }
  cout << ans * 6 + 1;
  return 0;
}
