1-2-3+4+5-6-7+8+9-10-11+12+···+2005-2006-2007+2008+2009
= (1-2-3+4)+(5-6-7+8)+(9-10-11+12)+···+(2005-2006-2007+2008)+2009
= 0×502 + 2009
= 2009