A snail crawls on the wall of a 10-foot-high well. It climbs 3 feet during the day and stops to rest at night. During the rest, because the well wall is too slippery, it always slides down 2 feet every night. How many days does it take for the snail to climb from the bottom of the well to the top of the well?
一只蜗牛在一面高10尺的井的壁上爬行，它白天爬高3尺，晚上停下来休息，休息时由于井壁太滑，每晚总要下滑2尺．则这只蜗牛从井底爬到井上需要多少天．
<gadget id="calculator">3 - 2</gadget>
<output>1</output>

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

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<result>8</result>