2002²-2001²+2000²-1999²+.+2²-1²
=(2002-2001)(2002+2001)+(2000-1999)(2000-1999)+...+(2-1)(2+1)
=2002+2001+2000+1999+.+2+1
=（2002+1)*2002/2
=2003*1001
=2005003