You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-ethyl-3-[(1R,2R,4S)-2-ethyl-2-(hydroxymethyl)-7-azabicyclo[2.2.1]heptane-7-carbonyl]-7-methyl-1,8-naphthyridin-4-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H27N3O3
assistant