Calculation: 2005*2004-2004*2003+2003*2002-2002*2001+...+3*2-2*1=how much.
计算：2005*2004-2004*2003+2003*2002-2002*2001+…+3*2-2*1=多少．
<gadget id="calculator">1_002 + 1</gadget>
<output>1_003</output>

<gadget id="calculator">1_003 / 2</gadget>
<output>1_003/2 = around 501.5</output>

<gadget id="calculator">2 * 2 * (1_003/2) * 1_002</gadget>
<output>2_010_012</output>

<result>2_010_012</result>