The diene shown reacts with maleic anhydride in a Diels-Alder reaction. How many stereocentres are present in the adduct (count every stereocentre irrespective of endo or exo orientation)? Give the answer as an integer.

The compound shown is (2E,4E)-hexa-2,4-diene. Its adduct with maleic anhydride is a 3,6-dimethylcyclohex-4-ene-1,2-dicarboxylic anhydride. The two ring carbons joined to the anhydride group and the two ring carbons carrying methyl groups each have four different substituents. That gives 4 stereocentres.