You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(2R)-2-(2,4-dichlorophenoxy)-1-[(7R)-1,1-dioxo-7-phenyl-1,4-thiazepan-4-yl]propan-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H21Cl2NO4S
assistant