You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4,7-dimethyl-6-(2-methylphenyl)-2-(2-methylpropyl)-4a,9a-dihydropurino[7,8-a]imidazole-1,3-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H25N5O2
assistant