In the illustration of a mathematics book, there are 100 parallelograms, 80 rectangles, and 40 rhombuses. What is the minimum number of squares in the illustrations in this book.
在一本数学书的插图中，有100个平行四边形，80个长方形，40个菱形．这本书的插图中正方形最少有多少．
<gadget id="calculator">40 - 100</gadget>
<output>-60</output>

<gadget id="calculator">80 + (-60)</gadget>
<output>20</output>

<result>20</result>