1-3+5-7+9-11.+2007-2008+2009-2011
=(1-3)+(5-7)+(9-11).+(2003-2005)+(2007-2008)+(2009-2011)
=(-2)+(-2)+(-2)+.+(-2)+(-2)+(-2)
=(-2)*[1+(2009-1)/4]
=-2-2008/2=-1006
1-3+5-7+9-11.+2007-2008+2009-2011
=(1-3)+(5-7)+(9-11).+(2003-2005)+(2007-2008)+(2009-2011)
=(-2)+(-2)+(-2)+.+(-2)+(-2)+(-2)
=(-2)*[1+(2009-1)/4]
=-2-2008/2=-1006