Use a rope to surround a big tree. If there are 3 meters left after winding 9 times, if there is a difference of 1 meter after winding 11 times, then how many meters will be left after winding 8 times.
用一根绳子围大树，如果绕9圈剩下3米，如果绕11圈差1米，那么，绕8圈则剩下多少米．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">11 - 9</gadget>
<output>2</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<gadget id="calculator">3 - 16</gadget>
<output>-13</output>

<gadget id="calculator">18 + (-13)</gadget>
<output>5</output>

<result>5</result>