You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[(6R)-6-(3-bromophenyl)-3-methylsulfanyl-6H-[1,2,4]triazino[5,6-d][3,1]benzoxazepin-7-yl]propan-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H17BrN4O2S
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