There is a rope 31.4 meters long. Xiao Zhang uses this rope to enclose a circular field on the playground for games. What is the area of this field?
有一根绳子长31.4米，小张用这根绳子在操场上围出一块圆形场地做游戏，这块场地的面积有多大？
<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>