Does 2x2 subtable mean a square made up of 4 unit squares, or the 4 intersection points of 2 columns and 2 rows (not necessarily adjacent)?

4 unit squares

When I wasn't know how to use the search function, I posted this problem here. Mathlover_SH gave a solution there.

The problem 6 of 2011 Cono Sur is a generalization of this problem: https://artofproblemsolving.com/community/c6h603541p3583778