The length of a rectangle is 80 meters and the width is 70 meters. Dig a square pool with a side length of 40 meters in the middle of it, and plant grass and greenery around it. How many square meters is the green area?
一块长方形地的长是80米，宽是70米．在它的中间挖一个边长40米的正方形水池，周围种草绿化．绿化部分的面积是多少平方米？
<gadget id="calculator">80 * 70</gadget>
<output>5_600</output>

<gadget id="calculator">40 * 40</gadget>
<output>1_600</output>

<gadget id="calculator">5_600 - 1_600</gadget>
<output>4_000</output>

<result>4_000</result>