frog jumps from the bottom of the river to the bank. The slope is 21 feet long. The frog jumps 3 feet each time and retreats 1 foot. How many times does the frog need to jump to reach the river bank?
一只青蛙从河底往岸上跳，坡长21尺，青蛙每次跳3尺，退1尺，青蛙上河岸最少需要跳多少次？
<gadget id="calculator">21 - 3</gadget>
<output>18</output>

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

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<result>10</result>