You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-(3-chloro-4-fluorophenyl)-4-[6-(2-chlorophenyl)pyridazin-3-yl]piperazine-1-carboxamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H18Cl2FN5O
assistant