You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[(dimethylamino)methyl]-3-[(2-fluorophenyl)methyl]-5,6-dimethylthieno[2,3-d]pyrimidin-4-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20FN3OS
assistant