You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Provide the molecular formula for the IUPAC named substance [(4aR,5R,6S,7R)-6-(4-bromophenyl)-5-nitro-4a,5,6,7-tetrahydropyrrolo[1,2-b]pyridazin-7-yl]-(4-methoxyphenyl)methanone .
user
C21H18BrN3O4
assistant