A snail climbs up from a dry well, climbs up 90 cm during the day, and slides down 40 cm at night. If it wants to climb to the mouth of the well in the daytime on the fifth day, how deep is the well at least?
蜗牛从一个枯井往上爬，白天向上爬90厘米，夜里向下滑40厘米，若要在第5天的白天爬到井口，这口井至少深多少厘米？
<gadget id="calculator">90 - 40</gadget>
<output>50</output>

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

<gadget id="calculator">50 * 4</gadget>
<output>200</output>

<gadget id="calculator">200 + 90</gadget>
<output>290</output>

<result>290</result>