Reversi problem #525

The task

To move

Light

Difficulty

Hard

Goal

only winning move

11 squares are still empty. With the right move the final count is 39 to 25; the other 6 moves do not win. Eleven empty squares and a single winning move. Expect a pass or a quiet move somewhere in the line.

What to look for

• The target is a corner.
• A pass occurs in the line, so the same side may move twice.
• A natural move only draws; you need the one that wins.
• 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.