3-5+7-9+11-13...+1995-1997+1999
=(3-5)+(7-9)+(11-13)...+(1995-1997)+1999
=-2 * 499 + 1999
=-2 * 500 + 2 + 1999
=-1000 + 1999 + 2
=999 + 2
=1001