Compute 1 - 2 + 3 - 4 +... + 2005 - 2006 + 2007.

(1 - 2) + (3 - 4) + (5 - 6) +.......= -1 + -1 + -1 + -1.......etc . for (2007 - 1) / 2 + 1 =-1 x 1004 terms =-1004

(1 - 2) + (3 - 4) + (5 - 6) + ... + (2003 - 2004) + (2005 - 2006) + 2007

=  -1 + -1 + -1 (for 1003 term) + 2007

=  -1003 + 2007  =  1004