Crystal Reports 2016

I'm working on report. I have the price list by item with date of price update.

I added the column calculating difference between the prices.

({ITEM_PRICE.PRICE_DISC}) - next({ITEM_PRICE.PRICE_DISC})

I've also group the report by items category. GroupName ({ITEM_PRICE.PRICE_LIST})

The problem is that the calculation formula is pulling data between groups. I need for this formula to work only inside the group. How can I do it?

Thanks