You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Under IUPAC nomenclature, a molecule named methyl (1R,3R,3aR,6aS)-1-(furan-2-yl)-3-methyl-5-(4-methylphenyl)-4,6-dioxo-1,2,3a,6a-tetrahydropyrrolo[3,4-c]pyrrol-2-ium-3-carboxylate is given. What is its corresponding molecular formula?
user
C20H21N2O5+
assistant