Summary report two tables running total

[ireckonso]

I have searched the forums, but haven't found an answer to this. I have a file with two tables, received and shipped. Have successfully created a report by month with total received and total shipped and the difference between the two. What I would like to do and cannot figure out is to have another field which keeps a running Balance on Hand as illustrated below. My results do not reflect this, not sure what I need to do at this point, attached is file. Thanks for any help and sorry if this has been answered before.

Month         Received      Shipped    Difference       Balance on Hand

Jan 2016     425               400                    25                    25    Can't get this column to report correctly

Feb 2016     370               350                   20                    45

Mar 2016     340               270                   70                  115

April 2016   310               400                   (90)                  25

 

BalanceonHand.fmp12