A dry well is 20 meters deep. A frog climbs up from the bottom of the well. It climbs up 5 meters during the day and slides down 2 meters at night. It will take at least how many days for the frog to climb out of the dry well.
一口枯井深20米，一只青蛙从井底向上爬，白天向上爬5米，晚上向下滑落2米，至少经过多少天，这只青蛙才能爬出枯井．
<gadget id="calculator">20 - 5</gadget>
<output>15</output>

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

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<result>6</result>