By how many centimeters is the perimeter of a square with side length 18 cm shorter than the perimeter of a square with side length 20 cm.
边长是18厘米的正方形的周长比边长是20厘米的正方形的周长短多少厘米．
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">18 * 4</gadget>
<output>72</output>

<gadget id="calculator">80 - 72</gadget>
<output>8</output>

<result>8</result>