Four in a row problem #521

The task

To move

Dark

Difficulty

Hard

Goal

win in 4

20 discs are already on the grid. Win in four: the threats have to be built in the right order so that every reply is forced.

What to look for

• Every move forces the reply, so the line can be calculated to the end.
• The winning line is diagonal, the direction players tend to miss.

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.