Take any 21 points on a segment (including both ends). How many line segments are there in total?
在一线段上任取21个点，(包括两端点)．则一共有多少条线段．
<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">21 * 10</gadget>
<output>210</output>

<result>210</result>