Problem 16 (2025 AMC 8)

Kei draws a $6 \times 6$ grid. He colors $13$ of the unit squares silver and the remaining squares gold. Kei then folds the grid in half vertically, forming pairs of overlapping unit squares. Let $m$ and $M$ be the least and greatest possible number of gold-on-gold pairs, respectively. What is $m+M$?