Calculation: 3-5+7-9+11-13+...+1995-1997+1999=how much.
计算：3-5+7-9+11-13+…+1995-1997+1999=多少．
<gadget id="calculator">1_995 - 1_997</gadget>
<output>-2</output>

<gadget id="calculator">(-2) + 1_999</gadget>
<output>1_997</output>

<gadget id="calculator">1_999 - 1_997</gadget>
<output>2</output>

<gadget id="calculator">2 * 499</gadget>
<output>998</output>

<gadget id="calculator">3 + 998</gadget>
<output>1_001</output>

<result>1_001</result>