You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-methyl-5-[(1S)-1'-(4-nitropyridin-2-yl)spiro[1,2-dihydroindene-3,4'-piperidine]-1-yl]-1,3,4-oxadiazole The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H21N5O3
assistant