There are 12 more books on the upper layer than on the lower layer. How many books must be moved from the upper layer to the lower layer to make the two layers have the same number of books.
上层书比下层书多12本，要从上层移多少本书到下层，才能使两层书一样多．
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>