You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 2-[1-(3-cyclopropyl-1,2,4-oxadiazol-5-yl)-2-methylbutyl]-3H-isoindol-1-one by IUPAC nomenclature, provide its molecular formula.
user
C18H21N3O2
assistant