A snail fell into a well 15 meters deep. It climbed up 3 meters every day during the day and slid down 1 meter at night. At this speed, how many days would it take for the snail to climb out of the well?
一只蜗牛掉在15米深的井中，它每天白天上爬3米，晚上滑下1米，按这样的速度，蜗牛要多少天才能爬出井口．
<gadget id="calculator">15 - 3</gadget>
<output>12</output>

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

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>