Sum textbox values in Reporting Services

I'm new to Microsoft's SQL Reporting Services, using Visual Studio to create reports.  Due to an unusual gathering of data, I have a need to create a Grand Total level textbox that sums the values of another textbox within a report (*NOT* values from a dataset field).  Either I don't know the syntax, or it doesn't seem to be possible.  I would want something like =Sum(textboxname), but that doesn't seem to be an option.  An alternative would be to create a variable that adds up textboxname each time it's displayed on the report, than display the variable in the Grand Total, but I don't see how to do that either.

Any help would be greatly appreciated.