You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
What is the molecular formula of the molecule [(4S,9aR)-4-methyl-3,4,6,7,9,9a-hexahydro-1H-pyrazino[2,1-c][1,4]oxazin-8-yl]-spiro[6,7-dihydrothieno[3,2-c]pyran-4,4'-piperidine]-2-ylmethanone ?
user
C20H29N3O3S
assistant