A piece of rectangular paper with a length of 30 cm and a width of 20 cm, on which the largest circle is cut. How many square centimeters of paper are left useless?
一张长30厘米，宽20厘米的长方形纸，在纸上剪一个最大的圆．还剩下多少平方厘米的纸没用？
<gadget id="calculator">30 * 20</gadget>
<output>600</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 ** 2</gadget>
<output>100</output>

<gadget id="calculator">3.14 * 100</gadget>
<output>314</output>

<gadget id="calculator">600 - 314</gadget>
<output>286</output>

<result>286</result>