snail has to climb a 15-meter-high wall, climb 5 meters and slide down 4 meters every day. How many days does it take to reach the top of the wall?
蜗牛要爬15米高的墙，每天爬5米滑下4米，要爬几天才能到墙顶？
<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">10 / 1</gadget>
<output>10</output>

<gadget id="calculator">10 + 1</gadget>
<output>11</output>

<result>11</result>