Crystal Reports Summarise fields

Message

   Hi, got a report and have to summarise a formula field which when i use sum option it's showing that it can't be summarized. Report is fetching information from database about how many products haven't been send to customer previous day(shorts) and compares it with our stock on each product to determin wheter production needs to produce some of that product or wehave stock on it and it can be dispatched straight away.The field i want to summarize is in the group footer and it's name is: shortstotal with a formula in it:
IF ({@brittotalbyproduct} + {@scototalbyproduct} - {usr_Report_Production_Shorts;1.qty}<=0)
then 
(
0
)
else
(
{@brittotalbyproduct} + {@scototalbyproduct} - {usr_Report_Production_Shorts;1.qty}
)

field. It basically checks wheter there is enough stock in the warehouse to dispatch it (if yes than shorts = 0 as production doesn't have to produce any of it to cover shorts, if not than it calculates how many of that product needs to be produced). It's in group footer because in details there is  few different fields which i use to determin wheter products a british or scottish and i can't move it into details area.

I know there is a 3 step formula which can be set to calculate but all examples i found didn't work for me or i'm doing something wrong so please help and give me some advice how to summarize it.