Plant trees around a square lawn, plant one tree at each corner, and plant 13 trees on each side. How many trees are planted around this lawn?
在一块正方形草地四周种树，四个角都种一棵，每边种13棵，这块草地四周共种树多少棵？
<gadget id="calculator">13 * 4</gadget>
<output>52</output>

<gadget id="calculator">52 - 4</gadget>
<output>48</output>

<result>48</result>