Complex sum

I have a report created using SQL Server Reporting tools.

The report contains a field 'FIELD A' which displays the difference of hours in that record.
At the end of the report, I need to sum all FIELD A and display in FIELD B.

How do I do it?One way to do this, if all the 'FIELD A' textboxes exist on one page, is to add the 'Field B' textbox to the Page Footer, and use the SUM aggregate over the textbox. For example,

=(Sum(ReportItems!FieldATextbox.Value))

This will only work if the expression is in the Page Header or Footer, and if all instances of the 'Field A' textbox is on the same page.

If not all instances will exist on one page, then you could use a simple custom function that is called from the 'Field A' textbox. This custom function adds the value passed to a member field, and then returns the value passed. In the 'Field B' textbox call a separate custom function that returns the value stored in the member field.

Ian