1 -2 + 3 - 4 + 5 -6 + 7... - 2004 + 2005 - 2006 + 2007 =

......................1 +(-2 + 3) + (- 4 + 5) + (-6 + 7)... +(- 2004 + 2005) + (- 2006 + 2007)

There are 1003 bracketed terms each of which has value 1, and an additional

1 at the beginning, so the sum is 1004.

RonL