21 students cross the river, there is only one small boat, and only 5 students can sit on the boat, how many times can they all cross the river at least.
21个同学过河，只有一条小船，船上只能坐5个同学，最少载多少次可以全部渡完．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

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

<result>5</result>