Four in a row problem #518

The task

To move

Light

Difficulty

Medium

Goal

win in 3

19 discs are already on the grid. Win in three: a forcing move first, then the double threat.

What to look for

• Every move forces the reply, so the line can be calculated to the end.
• The winning line runs along a row.

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.