You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
tert-butyl (3aR,10aS)-1,3-dioxo-3a,4,10,10a-tetrahydropyrrolo[3,4-b]carbazole-9-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C19H20N2O4
assistant