On the same plane, how many points of intersection can 4 different straight lines have at most?
同一平面上，4条不同的直线最多有多少个交点．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">(1/2) * 4 * 3</gadget>
<output>6</output>

<result>6</result>