need some more select help

I'm trying to

select a,count(1) FROM (select a from X UNION select a from Y ) group by a;

but it complains and says that the subquery must have an alias and I
have not figured it out how to add that.

Btw. is there any other way of doing what I'm doing above?

It's basically    select a,count(1) from X group by a     +  select
a,count(1) from Y  group by a  where  "+" means they should be summed
on the "a" field.

I tried
select a,sum(b)   from ( select a,count(1) as b from X group by a
union select a,count(1) as b from Y group by a) group by a

but that gave also an error asking for an alias for the subquery.

How can I solv this problem?