Formula

Hi guys,

MY clients can pay in 2 terms when ordering something from me.

I use formula to track how much they still have to pay me, but is missing the payment 2

What i need is:

17-8-17

Price = 10
Payment 1 = 5
Payment 2 = 0
Payment 2 day =
Need to be paid = 5 (Formula)

20-8-17

Price = 10
Payment 1 = 5
Payment 2 = 5
Payment 2 day = 20-8-17
Need to be paid = 0

Can someone help me with this? because i already have price-payment1 and it works. but if i try to put payment 2.. it stops working.