You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Can you give the molecular formula of the molecule 7-[(4-fluorophenyl)methyl]-1,3-dimethyl-8-(3-oxobutan-2-ylsulfanyl)-5H-purin-7-ium-2,6-dione ?
user
C18H20FN4O3S+
assistant