Reversi problem #517

The task

To move

Dark

Difficulty

Medium

Goal

only winning move

8 squares are still empty. With the right move the final count is 33 to 31; the other 6 moves do not win. Eight empty squares, and one move wins. Count parity: who gets the last move in each region?

What to look for

• The target is a corner.
• A pass occurs in the line, so the same side may move twice.
• Every other move fails, so check each candidate to the end.

How the answer is checked

There is exactly one first move that forces the result; any other first move allows the defence to escape. Moves that keep the forced result are accepted later in the line, and a move that loses it is taken back with a short note.