German mathematician Reiss once proposed such a math problem. The depth of the well is 22 feet. The snail is at the bottom of the well. It climbs 7 feet during the day and descends 2 feet at night. How many days can it reach the top of the well?
德国数学家里斯曾提出过这样一道数学题，井深22尺，蜗牛在井底，白天爬7尺，夜里降2尺，几天可以到达井顶？
<gadget id="calculator">22 - 7</gadget>
<output>15</output>

<gadget id="calculator">7 - 2</gadget>
<output>5</output>

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

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

<result>4</result>