Calculation: 2003-2002+2001-2000+1999-1998+1997=how much.
计算：2003-2002+2001-2000+1999-1998+1997=多少．
<gadget id="calculator">2_003 - 2_002</gadget>
<output>1</output>

<gadget id="calculator">2_001 - 2_000</gadget>
<output>1</output>

<gadget id="calculator">1_999 - 1_998</gadget>
<output>1</output>

<gadget id="calculator">1 + 1 + 1 + 1_997</gadget>
<output>2_000</output>

<result>2_000</result>