Consider the problem of building a wall out of 2×1 and 3×1 bricks (horizontal×vertical dimensions) such that, for extra strength, the gaps between horizontally-adjacent bricks never line up in consecutive layers, i.e. never form a "running crack".
For example, the following 9×3 wall is not acceptable due to the running crack shown in red:
<imgclass="img-responsive center-block"alt="9x3 wall with one lined up gap between horizontally-adjacent bricks"src="https://cdn.freecodecamp.org/curriculum/project-euler/crack-free-walls.gif"style="background-color: white; padding: 10px;">