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 1-(4,4-dimethyl-2-methylsulfonyl-1,3,6,7,9,9a-hexahydropyrazino[1,2-a]pyrazin-8-yl)-2-phenoxyethanone ?
user
C18H27N3O4S
assistant