Getting sum of Subreports, but its not subtracting the negative ch

[John C Brainard]

there are two sub reports they are part of the detail section of an energy
bill. it adds the positives, but doesn't subract the negatives...here is the
line

=[R-Detail Table subreport].Report![R-Detail3]+[N-Detail Table
subreport].Report![N-Detail3]

thanks for your help