Procedures Not Working

[Big K Jr]

I'm working with procedures for the first time in MySQL, but for some reason I keep getting NULL. My test procedure is a simple one, it just adds.

delimiter $$
create procedure adds(in r double, out a double)
begin
set a = r + r;
end $$
delimiter ;

CALL adds(5, @a);

SELECT @a;

Not sure if I'm doing this right. For @a it just prints out NULL.