Saw the largest circle from a rectangular wooden board with a length of 5 decimeters and a width of 4 decimeters. How many square decimeters is the remaining board?
从一个长5分米，宽4分米的长方形木板上锯下一个最大的圆，剩下的木板是多少平方分米？
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">3.14 * 4</gadget>
<output>12.56</output>

<gadget id="calculator">20 - 12.56</gadget>
<output>7.44</output>

<result>7.44</result>