Calculation: 3-4+5-6+...+2001-2002+2003-2004+2005.
计算：3-4+5-6+…+2001-2002+2003-2004+2005．
<gadget id="calculator">(-1) * 1_001</gadget>
<output>-1_001</output>

<gadget id="calculator">(-1_001) + 2_005</gadget>
<output>1_004</output>

<result>1_004</result>