You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Please write the molecular formula of the molecule ethyl (8S,11aR)-6,6-dioxo-8-phenyl-11,11a-dihydro-8H-pyrido[1,2-c][1,2,3]benzoxathiazine-9-carboxylate .
user
C20H19NO5S
assistant