Given a $$$n$$$ sized array, remove the elements of the array according to the following rules:

Exactly three elements are removed in one turn.

The removed three elements must be adjacent in the array, i.e., $$$arr[j]$$$, $$$arr[j+1]$$$, and $$$arr[j+2]$$$. And the second element must be $$$k$$$ greater than the first element, and the third element must be $$$k$$$ greater than the second element, i.e., $$$arr[j+1]-arr[j]=k$$$ and $$$arr[j+2]-arr[j+1]=k$$$.

Remove the elements from the array until no further triplets can be removed and return the size of the smallest possible such array.

Note: Return $$$-1$$$ if the array is empty.