You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[1-(4-chlorobenzoyl)-3-(3H-imidazo[4,5-c]pyridin-2-ylmethyl)piperidin-4-yl]acetic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H21ClN4O3
assistant