mysql count

I've no access to mysql NG, so I ask here.

I've now those 2 queries:

select count(IDAnnounce)as countarticles from XXX where DateTimeSell is NULL
select count(IDAnnounce)as countarticles from YYY where DateTimeSell is NULL

Now, I've to have an unique counter. I've tried a query, but I get more than
7800 results as I should get around 360

Now, how can I get the total amount of IDAnnounces from the differents
tables ???? with only 1 query ?