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Name the molecular formula for the molecule established as 2-bromoethyl (2S,5R,6R)-3,3-dimethyl-7-oxo-6-[(2-phenylacetyl)amino]-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylate using the IUPAC nomenclature.
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C18H21BrN2O4S
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