You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(7R)-3-(2-methoxyethyl)-9-(3-methoxyphenyl)-1,7-dimethyl-7,8-dihydro-6H-purino[7,8-a]pyrimidine-2,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H25N5O4
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