$5 \times 5$ square grid so that each number except 1 and 2, is the sum of two of its *neighbours*. Some of the numbers have already been filled in. Which number must replace the " $?$ " when the grid is complete.

**Note** - Numbers in the grid are *neighbours* if their squares touch along a side or at a corner. For example: the number "1" has eight *neighbours*, the number "21" has three *neighbours*.

**Bonus** - Complete the Grid.