(2001-1)+(2000-2)+(1999-3)+...+(1002-1000)
=(2001-1000)+(2000-999)+(1999-998)+...+(1002-1)
=1001+1001+1001+...+1001(共1000个1001)
=1001000
(2001-1)+(2000-2)+(1999-3)+...+(1002-1000)
=(2001-1000)+(2000-999)+(1999-998)+...+(1002-1)
=1001+1001+1001+...+1001(共1000个1001)
=1001000