#include <bits/stdc++.h>
using namespace std;
long long ans, n;
int main() {
  scanf("%I64d", &n);
  for (double i = 8.0 / 3; i < 2 * n / sqrt(3); i += 2) {
    double tmp = (sqrt(16.0 * n * n / 3 - 3 * i * i) - i) / 2.0 - 2.0 / 3;
    if (tmp > 0) ans += (long long)tmp / 2 + 1;
  }
  printf("%I64d\n", ans * 6 + 1);
  return 0;
}
