Compute 1 - 2 + 3 - 4 +... + 2005 - 2006 + 2007 - 2008 + 2009 - 2010.
(1 - 2) + (3 - 4) + .... + (2005- 2006) + (2007 -2008) + ( 2009 -2010)
We have (2010 / 2) = 1005 pairs each summing to -1
So
1005 (-1) = - 1005
