how to calculate difference from column totals (not rows)

[Paulius]

hi all.

have 2 columns , price calculation :

Sum gross / sum quantity.

in columns  Year 2018 another takes Y 2019.

so we can calculate difference, and in one difference column i use "Expression total", and get difference by row and average.

in another column difference i use "Sum of rows", and get sum.

then i can manually make difference from both totals , to get difference.

how to to it automatically ?

attaching example

[Paulius · Accepted solution]

solved. use aggregation in expression.