#include <bits/stdc++.h>
using namespace std;
int r;
bool check(int x, int y) { return 1ll * x * x + 3ll * y * y <= 4ll * r * r; }
int main(void) {
  scanf("%d", &r);
  int i = 1, j = 1;
  long long ans = 1;
  while (check(i, j + 2)) {
    ans += 2;
    j += 2;
  }
  do {
    i += 3;
    ++j;
    while ((j > 0) && ((!check(i, j)) || (!check(i + 1, j - 1)))) j -= 2;
    if (j > 0) ans += j * 2;
  } while (j > 0);
  printf("%I64d\n", ans);
  return 0;
}
