Sum column from multiple tables and diff SQL

Need to select multiple column from multiple tables sum and difference
 
Some help?

select a.naziv as 'Naziv artikla', SUM (cast(a.kolicina as float)) as 'Nabavka', SUM (cast(b.kolicina as float)) as 'Prodaja', b.roba as  'Naziv artikla2'  
FROM (a.naziv, a.kolicina from ulazni_racun_roba_roba as a union all select a.naziv, a.kolicina from mp_ulazni_racun_ostalo_roba as a union all select b.roba, b.kolicina from mp_racun_roba as b union all select b.roba, b.kolicina from mp_faktura_roba as b) as alltables  
select (a.kolicina-b.kolicina) as UKUPNO  
group by naziv