You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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4-[(2R)-2-(2,4-dichlorophenyl)-3-[(4-fluorophenyl)-hydroxymethylidene]-4,5-dioxopyrrolidin-1-yl]butanoate is the IUPAC name of a molecule. Please give its molecular formula.
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C21H15Cl2FNO5-
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