You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
methyl (1R,3R,3aR,6aR)-3-(hydroxymethyl)-5-methyl-1-(4-methylphenyl)-4,6-dioxo-1,2,3a,6a-tetrahydropyrrolo[3,4-c]pyrrol-2-ium-3-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C17H21N2O5+
assistant