You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4R,7aS)-1-(4-bromophenyl)-4-methyl-3-(4-methylphenyl)-5,6,7,7a-tetrahydropyrrolo[1,2-a]imidazol-4-ium is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H22BrN2+
assistant