Check whether 200+900=1100 is correct and how much can be used
验算200+900=1100是否正确，可用多少
<gadget id="calculator">200 - 200</gadget>
<output>0</output>

<gadget id="calculator">900 + 0</gadget>
<output>900</output>

<gadget id="calculator">200 - 900</gadget>
<output>-700</output>

<gadget id="calculator">900 + (-700)</gadget>
<output>200</output>

<result>200</result>