After a grenade explodes, the perimeter of the effective killing range is 31.4 meters. How many square meters is the effective killing area?
一种手榴弹爆炸后，有效杀伤范围周长是31.4米，有效杀伤面积是多少平方米？
<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>