#include <bits/stdc++.h>
double n;
double eps = 1e-6;
bool pd(double x, double y) {
  if (x * x + (y + 1) * (y + 1) - eps > n * n) return false;
  if (x * x + (y - 1) * (y - 1) - eps > n * n) return false;
  if ((x + sqrt(3) / 2) * (x + sqrt(3) / 2) + (y + 0.5) * (y + 0.5) - eps >
      n * n)
    return false;
  if ((x + sqrt(3) / 2) * (x + sqrt(3) / 2) + (y - 0.5) * (y - 0.5) - eps >
      n * n)
    return false;
  if ((x - sqrt(3) / 2) * (x - sqrt(3) / 2) + (y + 0.5) * (y + 0.5) - eps >
      n * n)
    return false;
  if ((x - sqrt(3) / 2) * (x - sqrt(3) / 2) + (y - 0.5) * (y - 0.5) - eps >
      n * n)
    return false;
  return true;
}
int main() {
  scanf("%lf", &n);
  long long l = 1, r = 2333333333ll, t = 0;
  while (l <= r) {
    long long m = (l + r) >> 1;
    if (pd(sqrt(3) * (m - 1), 0.0)) {
      t = m - 1;
      l = m + 1;
    } else {
      r = m - 1;
    }
  }
  long long ans = t;
  double y = -3.0;
  while (pd(0.0, y)) {
    long long l = 1, r = 2333333333ll, t = 0;
    while (l <= r) {
      long long m = (l + r) >> 1;
      if (pd(-sqrt(3) / 2 * (m - 1), y - (m - 1) * 1.5)) {
        t = m;
        l = m + 1;
      } else {
        r = m - 1;
      }
    }
    y = y - 3.0;
    ans += t * 2 - 1;
  }
  ans = ans * 6 + 1;
  printf("%lld\n", ans);
  return 0;
}
