(2011-2010+2009-2008+2007-…-2+1)/2.
(2011-2010+2009-2008+2007-…-2+1)/2．
<gadget id="calculator">1_005 + 1</gadget>
<output>1_006</output>

<gadget id="calculator">1_006 / 2</gadget>
<output>503</output>

<result>503</result>