1-2+3-4+5-6+...-2002+2003-2004

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1-2+3-4+5-6+...-2002+2003-2004
=(1-2)+(3-4)+(5-6)+...+(2001-2002)+(2003-2004)
=-1-1-1-1-.-1-1
=-1*1002
=-1002