A snail fell into a 9-meter-deep well. It climbed up 3 meters every day during the day and slid down 1 meter at night. How many days would it take to climb out of the well?
一只蜗牛掉进一口9米深的井里，它每天白天爬上3米，夜里又滑下1米，这样要多少天，才爬出井口．
<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>