Calculation: 20+19-18+17+16-15+...+2+1-0=how much.
计算：20+19-18+17+16-15+…+2+1-0=多少．
<gadget id="calculator">1 - 0</gadget>
<output>1</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">21 + 3</gadget>
<output>24</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">24 * (7/2)</gadget>
<output>84</output>

<result>84</result>