You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(5R)-3-[(3-butyl-1,2,4-oxadiazol-5-yl)methyl]-5-ethyl-5-(4-fluorophenyl)imidazolidine-2,4-dione The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H21FN4O3
assistant