There are 18 spiders, dragonflies, and cicadas in total, with 118 legs and 20 pairs of wings (spiders have 8 legs; dragonflies have 6 legs and two pairs of wings; cicadas have 6 legs and a pair of wings), how many dragonflies have Only.
有蜘蛛、蜻蜓、蝉三种动物共18只，共有腿118条，翅膀20对(蜘蛛8条腿；蜻蜓6条腿，两对翅膀；蝉6条腿，一对翅膀)，其中蜻蜓有多少只．
<gadget id="calculator">18 * 6</gadget>
<output>108</output>

<gadget id="calculator">118 - 108</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">18 - 5</gadget>
<output>13</output>

<gadget id="calculator">13 * 1</gadget>
<output>13</output>

<gadget id="calculator">20 - 13</gadget>
<output>7</output>

<result>7</result>