Two straight lines have at most 1 intersection, three straight lines have at most 3 intersections, and five straight lines have at most how many intersections.
两条直线最多有1个交点，三条直线最多有3个交点，五条直线最多有多少个交点．
<gadget id="calculator">1 + 2 + 1 + 2 + 4</gadget>
<output>10</output>

<result>10</result>