For example, the region consisting of {2,3,8} is a valid region. It contains 3 squares (3 is an odd prime) and the sum of the numbers in the squares is prime (2+3+8=13 is an odd prime).

+--+--+--+--+--+ | 1| 2| 3| 4| 5| +--+--+--+--+--+ | 6| 7| 8| 9|10| +--+--+--+--+--+ |11|12|13|14|15| +--+--+--+--+--+ |16|17|18|19|20| +--+--+--+--+--+ |21|22|23|24|25| +--+--+--+--+--+