You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(5R)-2-N-methyl-4-N-(1-methylcyclopropyl)-2-N-[2-(oxan-4-yl)ethyl]-5-oxo-6,7-dihydrothieno[3,2-d]pyrimidine-2,4-diamine The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H28N4O2S
assistant