You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[2-[1-[2-(2-chloro-4-fluorophenyl)acetyl]piperidin-3-yl]imidazol-1-yl]acetamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H20ClFN4O2
assistant