You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(2,3-dihydro-1H-inden-1-yl)-1-(1-methyl-1,4,9-triazaspiro[5.5]undecan-4-yl)ethanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H29N3O
assistant