Grand total for formula fields?

[janis]

Hi,
in report I have group and in group footer formula field (@f1):

if sum(detail.field)>0 then
     sum(detail.field)
else
     0

Now I can't get grand total of @f1, I get error 'This filed cannot be summarized.'.
In this report I need to get grand total, but if sum(detail.field) in group is less then zero in grand total I need to add zero not to add negative value.
Did anybody know how to do this?

regard j