You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[(4aR,7aR)-1-(5-acetamido-2-fluorophenyl)-4,4a,5,6,7,7a-hexahydropyrrolo[3,4-d][1,3]thiazin-1-ium-2-yl]benzamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H22FN4O2S+
assistant