$$$$Calculate$$$$  $$\frac{1}{2}~ (2019 \times 2018-2018 \times 2017+2017 \times 2016-2016 \times 2015+\cdots$$$$ +5\times4-4\times3 +3\times2-2\times1 )$$． 
Overseas Competition->Knowledge Point->Calculation Modules->Whole Numbers->Mixed Operations
$$\dfrac{1}{2}(2019\times2018-2018\times2017+2017\times2016-2016\times2015+\cdots$$$$+5\times4-4\times3+3\times2-2\times1)$$  $$= \frac{1}{2}\left (2 \times 2018+2 \times 2016+ \cdots +2 \times 4+2 \times 2\right )$$  $=\left (2018+2016+\cdots+4+2\right )=1009\times1010=1019090$. 