Google Drive Sheet - Simple Formula Help

Folks, I could really use some help with a simple doc I'm setting up to track expenses and payments between two roommates. For the purpose of solving this problem, there will always be just two roommates in this doc. In short, I just need it to work as you see it.

It is public, and located here:

https://docs.google.com/spreadsheet/...XlBR2Nub19tbFE

As you can see, I simply need one column to track one person's expenses, and another to track the other's payments, and an area that does the math. But you can see that it's not working as I need it to, because the balance for 'Dennis Owed' should be zero.

Would someone be able to help me out with this?

Greatly appreciated!!!

This is an EXCEL forum, and you can attach workbooks directly to your post, so why should we follow a link to a Google spreadsheet?

Pete