Or for olympiad for 13-year-old codes - it's taken from this year Ukrainian Olympiad of Informatics, 2nd stage (1st - school, 2nd - town, 3rd - region, 4th - republic stage). Pupils 7-11 grade take part, it means participants are 12-17-year-old. I proposed 6 kyu, please propose your own level :)

Reduced number of tests (it was 100) and now tests load takes 1.3 sec and tests load plus solution takes 1.6 sec

Added. Hope this will be understandable

Already have in description:

Let H(n) be the number of all regular hexagons that can be found by connecting 6 of these points.

Searching 12th hexagon is way to solution

Done

Fixed

Fixed, please check

nice

This is very minor issue:
using namespace std;
is required and after removing this line testing code fail with compilation error.
This is not good, because e.g. I never use using namespace std;.
Better to let decide use this or not.
Thanks.