There is a column of numbers 3, 7, 11, ..., 643, what is the average number of this column.
有一列数3、7、11、…、643，这列数的平均数是多少．
<gadget id="calculator">3 + 643</gadget>
<output>646</output>

<gadget id="calculator">646 / 2</gadget>
<output>323</output>

<result>323</result>