On the number line, -2 is the same distance from 0 as how much is from 0.
在数轴上，-2到0距离和多少到0的距离一样．
<gadget id="calculator">(-2) - 0</gadget>
<output>-2</output>

<gadget id="calculator">0 - (-2)</gadget>
<output>2</output>

<result>2</result>