Calculation: (2+4+6+...+996+998+1000)-(1+3+5+...+995+997+999)=how much.
计算：(2+4+6+…+996+998+1000)-(1+3+5+…+995+997+999)=多少．
<gadget id="calculator">1_000 - 999</gadget>
<output>1</output>

<gadget id="calculator">1 * 500</gadget>
<output>500</output>

<result>500</result>