Using rSum To Add Multiple Cell Groups

I know I've been on here a lot as of late, but I cannot find the answer to this issue anywhere. I don't think it's a difficult one, but I can't seem to figure it out nonetheless:

I'm trying to create a formula that adds Cells A1 through A10 as well as Cells C1 through C10 and displays the sum into Cell E10. So, E10 should contain the formula =rSum(A1:A10,C1:C10), right? Wrong! If it was just a plain, simple old =Sum, it would work, but no, I need a difficult =rSum for some bizarre reason (actually because of this reason), and I just don't know how to work with the whole r-thing.

Anyone here familiar with =rSum?