Plant trees around a square site, with 15 trees on each side, and one tree at each of the four vertices. How many trees are planted around the site?
在一块正方形场地四周种树，每边都钟15棵，并且四个顶点都种有1棵，这块场地四周共种了多少棵树．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 - 4</gadget>
<output>56</output>

<result>56</result>