You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3R,4S)-3-methoxy-4-[1-(2-phenoxyacetyl)piperidin-4-yl]-1-propan-2-ylazetidin-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H28N2O4
assistant