You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-[6-[(3-amino-7-oxa-1-azaspiro[4.4]nonan-1-yl)methyl]-3-oxo-1H-isoindol-2-yl]piperidine-2,6-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H26N4O4
assistant