I have a spreasheet that contains data for 3 years

Jan 2006 Jan 2007 Jan 2008
50 50 0

Etc. Etc

All the months are populated for 2006 & 2007

But 2008 only has cells populate to June

I would like to sum only the rows for 2006 & 2007 up to the same YTD as 2008

The columns are there for 2008 there are just no numbers populated.

Is this possible?