TETRIS

Rishabh has an art object in his room with $H$ rows and $W$ columns, made up of $H×W$ blocks. Each block has a color represented by a lowercase English letter (a-z). The color of the block at the $i-th$ row and $j-th$ column is $c(i,j)$. Rishabh would like to dismantle the object, finding it a bit kitschy for his tastes. The dismantling is processed by repeating the following operation: Choose