You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(6-chloro-2-oxo-3H-benzimidazol-1-yl)-N-[(2R)-1'-methyl-2',4'-dioxospiro[1,3-dihydroindene-2,5'-imidazolidine]-5-yl]acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H18ClN5O4
assistant