1+2-3-4+5+6-7-8+9...+2005+2006-2007-2008+2009
=(1-3) + (2-4) + (5-7) +...+ (2006-2008) + 2009
= -2 * 2008/2 + 2009
= -2008 + 2009
= 1
1+2-3-4+5+6-7-8+9...+2005+2006-2007-2008+2009
=(1-3) + (2-4) + (5-7) +...+ (2006-2008) + 2009
= -2 * 2008/2 + 2009
= -2008 + 2009
= 1