Summing specific cells on two worksheets
 

Hi,

I'm working with a license purchase history and have multiple tabs
representing each year with one tab being a "summary" of the total number of
purchased licenses. I have already hand counted the total of all worksheets
and entered the numbers manually on the summary sheet. Going forth, I would
like the summary sheet to update automatically whenever a license is added to
the current year tab. So, for example, if the total number of licenses on
the summary worksheet for a particular product is 7, and I add 2 more to the
current year worksheet, the summary should now show 9 as the total. This
seems simple but I'm having a hard time with the formula. Here's what I use
but it will not work.
Choose the cell on the summary sheet and type the following:
=SUM('Summary!B2,'LicensePurchase7!N2) Where N2 is the total for the
specific year. I've also tried different variations of this but cannot get
it to work.

Any help is appreciated!
Thanks,
SP