There is a snail climbing up a 15-meter-high tree trunk. It climbs up 4 meters during the day and retreats 3 meters at night. How many days can the snail climb to the top of the tree?
有一只蜗牛沿15米高的树干往上爬，白天向上爬4米，夜间又往下退了3米，蜗牛第几天能爬到树顶？
<gadget id="calculator">15 - 4</gadget>
<output>11</output>

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

<gadget id="calculator">11 / 1</gadget>
<output>11</output>

<gadget id="calculator">11 + 1</gadget>
<output>12</output>

<result>12</result>