You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name 1-ethyl-2-[2-[4-(3-ethyl-1,2,4-oxadiazol-5-yl)phenyl]ethyl]-3-(2-pyrrol-1-ylethyl)guanidine into its molecular formula.
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C21H28N6O
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