Calculate cumulative Total

The following is designed as a repeating subform with new row instances added via an Add button.

I need each Cumulative Payment field (on each row) to auto-calculate based on either:

1. the "current" and "all previous" payment amounts (i.e. cumulative payment 3 ($1,800) = sum of payments 1, 2, and 3)

or

2. the previous cumulative payment + current payment (i.e. cumulative payment 3  ($1,800) = cumulative payment 2 ($1,100) + payment 3  ($700)

Period    Payment     Cumulative Payment

     1.          500               500

     2.          600               1,100

     3.          700               1,800

Can someone please assist. I understand I need to grab the"previous instance(s)" put not sure what I'm doing wrong...