1-2+3-4+5-6+.2005-2006+2007-2008+2009
=（1-2）+（3-4）+（5-6）+.(2005-2006)+(2007-2008)+2009
=-1-1-1-...-1+2009
=-1x2008/2+2009
=-1004+2009
=1005