You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(2,2-difluoroethyl)-4a,5,5-trimethyl-6-oxo-1,3,4,8a-tetrahydroisoquinoline-7-carbonitrile is the IUPAC name of a molecule. Please give its molecular formula.
user
C15H20F2N2O
assistant