There are 20 coins of 1 cent, 2 cents and 5 cents, with a total value of 0.50 yuan. Among them, there are at least how many coins of 2 cents.
1分、2分、5分的硬币共20枚，总值0.50元，其中2分的硬币至少有多少枚．
<gadget id="calculator">4 * 7</gadget>
<output>28</output>

<gadget id="calculator">30 - 28</gadget>
<output>2</output>

<result>2</result>