Calculation: 4*5+5*6+6*7+...19*20=how much.
计算：4*5+5*6+6*7+…19*20=多少．
<gadget id="calculator">19 + 1</gadget>
<output>20</output>

<gadget id="calculator">19 * 2</gadget>
<output>38</output>

<gadget id="calculator">38 + 1</gadget>
<output>39</output>

<gadget id="calculator">39 / 6</gadget>
<output>13/2 = around 6.5</output>

<gadget id="calculator">19 * 20 * (13/2)</gadget>
<output>2_470</output>

<gadget id="calculator">1 * 1</gadget>
<output>1</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">2_470 - 1 - 4 - 9</gadget>
<output>2_456</output>

<gadget id="calculator">4 + 19</gadget>
<output>23</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">23 * 8</gadget>
<output>184</output>

<gadget id="calculator">2_456 + 184</gadget>
<output>2_640</output>

<result>2_640</result>