Subtraction formula issue

I have tried all previous suggestions and cannot get my subtraction formula to work.

I'm entering the following:

var a = this.getField("CurrentFeesPaid").value; // variable for the Current value

var b = this.getField("Totalmonthlypaid").value; // variable for the Total value

event.value = a - b; // compute difference of the two values

I want to subtract Total monthly paid from Current Fees Paid. I've tried simple and I've tried custom and nothing seems to actually calculate. In total monthly paid there is a formula adding several fields together. Please help.