$$(100 + 98 + 96 + \cdots +2) -(99 + 97 + 95 + \cdots + 1)=$$． 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Whole Numbers Addition and Subtraction 
$$(100-99)+(98-97)+ \cdots +(2-1) = 1+1+ \cdots +1 = 50$$. 