A square paper with a side length of 20 cm, cut a largest circle on the paper. How many square centimeters of paper are left useless?
一张边长为20厘米的正方形纸，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">20 * 20</gadget>
<output>400</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">400 - 314</gadget>
<output>86</output>

<result>86</result>