There is a well with a depth of 9 meters. A snail and a tortoise climb up from the bottom of the well at the same time. Because the well wall is slippery, the snail climbs up 2 meters during the day and slides down 1 meter at night; the tortoise climbs up 3 meters during the day and slides down 1 meter at night. When the tortoise climbed to the mouth of the well, how far was the snail from the mouth of the well?
有一口9米深的水井，蜗牛和乌龟同时从井底向上爬．因为井壁滑，蜗牛白天向上爬2米，晚上向下滑1米；乌龟白天向上爬3米，晚上向下滑1米．当乌龟爬到井口时，蜗牛距井口多少米．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

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

<gadget id="calculator">1 + 1 + 1 + 2</gadget>
<output>5</output>

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

<gadget id="calculator">2 + 2 + 2 + (-2)</gadget>
<output>4</output>

<result>4</result>