You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
tert-butyl (3S,9aS)-3-(4-isocyano-3-methylphenyl)-3,4,6,7,9,9a-hexahydro-1H-pyrazino[2,1-c][1,4]oxazine-8-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H27N3O3
assistant