A book has three volumes: first, middle, and lower. The first volume is 1 yuan more than the middle volume, the middle volume is 2 yuan more than the second volume, and the first volume is more than the second volume.
一部书有上、中、下三册，上册比中册多1元，中册比下册多2元，上册比下册多多少元．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<result>3</result>