Sales Returns - How Do I Make Them Negative On A Report

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Hi 

I have wrote a report which gives me sales by product, country's, analysis codes etc and is pulling through our w/o cost. 

However I am really struggling with the logic of how I can turn sales returns into a negative number. Does anyone know the script?

To get the sales value reported I am using the field SOPInvoiceCredits.BaseInvoicedNetValue

I have tried a number of scripts but none of them seems to work. All I want to do is to turn this field into a negative if the SOP order type is Sales Returns. Any ideas?

Thanks in advance!