A snail climbs up a 20-meter-high bamboo pole, climbs up 5 meters during the day, and slides down 2 meters at night. How many days can the snail climb to the top of the bamboo pole?
一只蜗牛沿着20米高的竹竿向上爬，白天向上爬5米，夜里向下滑2米，这只蜗牛第多少天可以爬到竹竿的顶端．
<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<result>6</result>