You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
7-chloro-1-oxo-N-(1,2,3,4-tetrahydroisoquinolin-5-ylmethyl)-2H-isoquinoline-3-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H18ClN3O2
assistant