Formula - Subtracting Totals...Help

I have a Crystal Report here. Version 9.

Basically have 2 different Charge Types, I am trying to subtract a Totals. One is an AP and one is an AR. Its pretty straight forward. I am not sure how to insert the correct formula to get them to subtract eachother. I imagine it goes in the Order Number footer.

The AP and AR Charge Amount share the the same field and are specified by another field called charge code.

see attchements

There are probably a few ways to do this, here is one

Create a Formula with this code

Code:

---

If {ChargeCode} = "AR" Then
  {ChargeAmount}
Else
  -{ChargeAmount}

---

Put the formula in the details section but suppress it. Then do a typical summary on the formula to display the result in the group footer.

Another option would be to create a running total field.