You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3S)-5-chloro-3-hydroxy-1-methyl-3-[2-oxo-2-(2,4,6-trimethylphenyl)ethyl]indol-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H20ClNO3
assistant