$$2001 + (2000 - 1999 + 1998 - 1997 + 1996 - \cdots + 2 - 1) =$$． 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Whole Numbers Addition and Subtraction 
$$2001+(2000-1999)+(1998-1997)+\cdots +(2-1)=2001+(1000$$ones$$)$$. 