Two points can determine a line segment, take 20 points on a straight line, how many line segments can be determined at most.
两点可以确定一条线段，在一条直线上取20个点，最多可以确定多少条线段．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">20 * 19</gadget>
<output>380</output>

<gadget id="calculator">380 / 2</gadget>
<output>190</output>

<result>190</result>