my solution is the screenshot below.
it's fair and hidden. very interesting.
but kuchiwo found another solution. it's seems unfair to me.
i want to know the intended solution.
i thought it over again. i was wrong to judge it unfair. it's a great game no matter which one is intended.
Kuchiwo's solution is intended, but seeing yours, I can't help but accept it as a perfectly valid alt solution; after all, it uses a trick just as clever.