toad fell into a well with a depth of 18 meters. It climbed 3 meters during the day and slid down 2 meters at night. How many days would it take to reach the mouth of the well?
一只蛤蟆掉在井里，井深18米，它白天爬3米，夜里滑下2米，爬到井口要用多少天．
<gadget id="calculator">18 - 3</gadget>
<output>15</output>

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

<gadget id="calculator">15 / 1</gadget>
<output>15</output>

<gadget id="calculator">15 + 1</gadget>
<output>16</output>

<result>16</result>