You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[(2S)-1-[2-(4-chlorophenyl)ethylamino]-3-methyl-1-oxobutan-2-yl]-2,6-difluorobenzamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H21ClF2N2O2
assistant