The tree-planting group planted trees around a square lawn, requiring one tree to be planted at each of the four vertices, and only 20 trees were planted on each side. How many trees were planted in total?
植树小组在一个正方形草坪的四周栽树，要求四个顶点都栽一棵，并且每边都只栽20棵，一共栽树多少棵．
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 - 4</gadget>
<output>76</output>

<result>76</result>