You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-[[2-[4-chloro-1-(4-chlorobenzoyl)-5-hydroxy-2-methylindol-3-yl]acetyl]amino]propanoic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H18Cl2N2O5
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