You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4-chloro-5-[4-(5-fluoro-2-nitrophenyl)piperazin-1-yl]-2-phenylpyridazin-3-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H17ClFN5O3
assistant