You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[3-fluoro-4-[4-[[5-(4-fluorophenyl)-1,3,4-oxadiazol-2-yl]methyl]piperazin-1-yl]phenyl]ethanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H20F2N4O2
assistant