The school bought a new batch of books, numbered from 420 to 640, how many books did the school buy in total?
学校新买来一批图书，编号从420号到640号，则学校一共买了多少本书．
<gadget id="calculator">640 - 420</gadget>
<output>220</output>

<gadget id="calculator">220 + 1</gadget>
<output>221</output>

<result>221</result>