平方差
原式=(2004+2003)(2004-2003)+(2002+2001)(2002-2001)+……+(2+1)(2-1)
=(2004+2003)×1+(2002+2001)×1+……+(2+1)×1
=2004+2003+2002+2001+……+2+1
=(2004+1)+(2003+2)+……+(1003+1002)
=2005×1002
=2009010