A piece of rectangular paper with a length of 50 cm and a width of 40 cm, and cut the largest circle on the paper, how many square centimeters of paper are left useless?
一张长50厘米，宽40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有用？
<gadget id="calculator">50 * 40</gadget>
<output>2_000</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

<gadget id="calculator">3.14 * 400</gadget>
<output>1_256</output>

<gadget id="calculator">2_000 - 1_256</gadget>
<output>744</output>

<result>744</result>