You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule 6-ethyl-N-[[1-[(4-fluorophenyl)methyl]piperidin-4-yl]methyl]thieno[2,3-d]pyrimidin-4-amine into molecular formula.
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C21H25FN4S
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