Draw 6 straight lines on a piece of paper, how many points of intersection can there be at most?
一张纸上画6条直线，最多有多少交点．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 + 2 + 3 + 4 + 5</gadget>
<output>15</output>

<result>15</result>