You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-butyl-2-[(2Z)-2-[(2-chloro-6-fluorophenyl)methylidene]-3-oxo-4a,5,6,7,8,8a-hexahydrobenzo[b][1,4]oxazin-4-yl]acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H26ClFN2O3
assistant