You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(5R,9R)-2,9-dimethyl-7-(6-methylpyridin-2-yl)-2,7-diazaspiro[4.4]nonan-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C15H21N3O
assistant