You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(4-bromophenyl)-2-(4-methoxyphenyl)-6,7,8,9-tetrahydro-5H-imidazo[1,2-a]azepin-4-ium;bromide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H22Br2N2O
assistant