A piece of grass with a side length of 10 meters has a tree at the middle point, and a sheep is tied to the tree with a rope of 5 meters. How many square meters is the grass that the sheep cannot eat?
一块边长是10米的正方形草地，在中点有一棵树，树旁栓一只羊，羊绳子5米，这只羊不能吃到的草地面积为多少平方米？
<gadget id="calculator">10 * 10</gadget>
<output>100</output>

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

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

<gadget id="calculator">100 - 78.5</gadget>
<output>21.5</output>

<result>21.5</result>