#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-9;
inline double f(long long a, long long k) {
  if (k < a) return 0;
  return (sqrt(k * k - a * a));
}
int main(void) {
  long long a, i, k, res = 0, r;
  scanf("%I64d", &k);
  k = k * 2;
  a = -2;
  r = (f(a + 3, k) + eps) / sqrt(3);
  r = (((r) < ((f(a + 4, k) + eps) / sqrt(3) + 1))
           ? (r)
           : ((f(a + 4, k) + eps) / sqrt(3) + 1));
  if (r % 2 == 0) r--;
  res = res + r;
  i = 1;
  while (r > 0) {
    a += 3;
    r = (int)((f(a + 3, k) + eps) / sqrt(3));
    r = (((r) < ((int)((f(a + 4, k) + eps) / sqrt(3) + 1)))
             ? (r)
             : ((int)((f(a + 4, k) + eps) / sqrt(3) + 1)));
    if ((r + i) % 2 == 0) r--;
    if (r > 0) res += 2 * r;
    i++;
  }
  printf("%I64d\n", res);
  return 0;
}
