Add a Summary to group Header

Hi to all:

I would Like add a summary field,in group header, in my report,that sum all values that have the number "1"...
This is Possible...Please see the image bellow...

The desired values,for the first group are:

-----------------------------Valores Mensais
---------------------------Débito--------Crédito
9001 - Custos Indirectos - 3.025,63-------500,00

Thanks