sticks are needed to place 1 triangle, 5 sticks are needed to place 2 triangles, 7 sticks are needed to place 3 triangles, how many sticks are needed to place n triangles.
摆1个三角形需要3根小棒，摆2个需要5根小棒，摆3个需要7根小棒，摆n个三角形需要多少根小棒．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

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

<result>3</result>