There are 18 spiders, dragonflies, and cicadas, with 118 legs and 20 pairs of wings. How many dragonflies are there? (Spider. 8 legs; Dragonfly 6 legs, 2 pairs of wings; Cicada 6 legs, 1 pair of wings)
有蜘蛛、蜻蜓、蝉三种动物共18只，共有腿118条，翅膀20对．问蜻蜓有多少只？(蜘蛛.8条腿；蜻蜓6条腿，两对翅膀；蝉6条腿，一对翅膀)
<gadget id="calculator">6 * 18</gadget>
<output>108</output>

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

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

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

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

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

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

<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">7 / 1</gadget>
<output>7</output>

<result>7</result>