Cumulative sum with calculated values

Hi !

I have a report that shows this

REG   DIF

1          4

2          8

3          9

and so on…

But DIF value is comming from a calculated value [Sum(REG)].  

I want to calculate cumulative sum in another column 

REG    DIF    CUM

1          4         4

2          8         12

3          9         21

Do you know any expression that can help me with this, if is possible.

I have my doubts because is a calculated field .

Help!