19*18-18*17+17*16-16*15+...+3*2-2*1=how much.
19*18-18*17+17*16-16*15+…+3*2-2*1=多少．
<gadget id="calculator">18 + 2</gadget>
<output>20</output>

<gadget id="calculator">9 / 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">20 * (9/2)</gadget>
<output>90</output>

<gadget id="calculator">2 * 90</gadget>
<output>180</output>

<result>180</result>