You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-[4,7-difluoro-2-(4-fluorophenyl)-1H-indol-3-yl]-N-[(3S)-2-oxooxolan-3-yl]propanamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H17F3N2O3
assistant