You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 1-[4-[4-[(4,5-dimethoxy-2-nitrophenyl)methyl]piperazin-4-ium-1-yl]phenyl]ethanone by IUPAC nomenclature, provide its molecular formula.
user
C21H26N3O5+
assistant