A cylinder has a perimeter of 31.4 decimeters. What is the cross-sectional area of this cylinder?
一根圆柱，外围周长是31.4分米，这根柱子的横截面积是多少平方分米？
<gadget id="calculator">31.4 / 3.14 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<result>78.5</result>