#include <bits/stdc++.h>
using namespace std;
double R;
double g3 = sqrt(3.0);
long long pd(long long i) {
  long long ret = 0;
  double tru1, tru2, tmp;
  {
    tmp = (double)(i * 3 - 2) * 0.5;
    tru1 = sqrt(R * R - tmp * tmp);
    if (R < tmp) return 0;
    tmp += 0.5;
    if (R < tmp) return 0;
    tru2 = sqrt(R * R - tmp * tmp);
  }
  if (i & 1) {
    ret = (long long)floor(tru2 / g3) * 2ll + 1ll;
    if (tru1 * 2.0 < g3) return 0;
    tru1 -= g3 / 2.0;
    ret = min(ret, (long long)floor(tru1 / g3) * 2ll + 1ll);
  } else {
    ret = (long long)floor(tru1 / g3) * 2ll;
    if (tru2 * 2.0 < g3) return 0;
    tru2 -= g3 / 2.0;
    ret = min(ret, (long long)floor(tru2 / g3) * 2ll + 2ll);
  }
  return ret;
}
int main() {
  scanf("%lf", &R);
  long long i = 1, ans = 0, las = pd(i);
  ans = las;
  while (las) {
    i++;
    las = pd(i) * 2ll;
    ans += las;
  }
  printf("%I64d", ans);
  return 0;
}
