Using DDE linked Cells

I am using DDE to pull in stock quotes and volume into an Excel 2007 spreadsheet.

I want to accumulate a running total of one of the volume fields. This field is constantly changing.

Let's say cell A1 contains the DDE linked volume data.

What forumula do I use in cell B1 to get a running total of each volume number that comes thru cell A1?

I know DDE is outdated...but that is the only interface they offer!

Thanks,
Rick