Calculation: 2+4+6+8+...+40+38+36+...+4+2.
计算：2+4+6+8+…+40+38+36+…+4+2．
<gadget id="calculator">2 + 38</gadget>
<output>40</output>

<gadget id="calculator">19 / 2</gadget>
<output>19/2 = around 9.5</output>

<gadget id="calculator">40 * (19/2) * 2</gadget>
<output>760</output>

<gadget id="calculator">760 + 40</gadget>
<output>800</output>

<result>800</result>