After publishing my solution, I found out that my solution has a bug.

  08-09
 /   /
12 13     # concave hexagon
 \  \
  18-19

`checkio([8,9,12,13,18,19])` should return `0`.

There are a few published solutions which returns `6` (like my solution).

Can you add these tests?

assert checkio([8,9,12,13,18,19]) == 0
assert checkio([8,10,17,19,32,34]) == 0