$$100-99+98-97+96-95+\cdots +4-3+2-1$$. 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Whole Numbers Addition and Subtraction 
$$\begin{eqnarray}\&\&\left( 100-99 \right)+\left( 98-97 \right)\cdots +\left( 4-3 \right)+\left( 2-1 \right)\textbackslash\textbackslash{} \&=\&50\times 1\textbackslash\textbackslash{} \&=\&50．\end{eqnarray}$$ 