How to make a "smart" sum in Microsoft Access?

Question

SQL

Comrades, we really need help. Essence in the picture:

There is a Docs table. How to make SQL-query (report, whatewer) returning the whole table plus one more column of "smart" running sum?
The running amount is the sum of the previous running amount, plus the value of Cost if it is a payment; minus Cost if this is a requirement; or, if document type: opening balance, then the value of the running amount becomes simply Cost.
You know, these things are easy to do in Excel.