You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-(4-amino-6-methylthieno[2,3-d]pyrimidin-5-yl)phenyl]-3-(3-methylsulfanylphenyl)urea is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H19N5OS2
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