You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4aR,8aR)-1-ethyl-N-[(1-methylbenzimidazol-2-yl)methyl]-2,2-dioxo-4,5,6,7,8,8a-hexahydro-3H-benzo[c]thiazine-4a-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H28N4O3S
assistant