difference in a multilevel crosstab ( mutligroup)

[NIVERSKY]

hi all , 

i have a crosstab with 3 groups ( Category,product ,year ) with 1 column ( months) and a summerised value of sales qty , 

i want to calculate the difference between the sales of the year 2024 and the year 2023  of every PRODUCT by months .

i tried several things without success .

thansk in advance