Calculation: 1999*1998-1998*1997-1997*1996+1996*1995.
计算：1999*1998-1998*1997-1997*1996+1996*1995．
<gadget id="calculator">1_999 * 1_998</gadget>
<output>3_994_002</output>

<gadget id="calculator">1_998 * 1_997</gadget>
<output>3_990_006</output>

<gadget id="calculator">1_997 * 1_996</gadget>
<output>3_986_012</output>

<gadget id="calculator">3_994_002 - 3_990_006 - 3_986_012</gadget>
<output>-3_982_016</output>

<gadget id="calculator">1_996 * 1_995</gadget>
<output>3_982_020</output>

<gadget id="calculator">(-3_982_016) + 3_982_020</gadget>
<output>4</output>

<result>4</result>