A circular piece of cardboard has an inner radius of 4 decimeters and an outer radius of 7 decimeters. What is the area of the circular cardboard in square meters?
一块环形纸板，内圆半径是4分米，外圆半径是7分米．环形纸板的面积是多少平方．
<gadget id="calculator">7 ** 2</gadget>
<output>49</output>

<gadget id="calculator">3.14 * 49</gadget>
<output>153.86</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<gadget id="calculator">153.86 - 50.24</gadget>
<output>103.62</output>

<result>103.62</result>