Plant trees around a square with a circumference of 900 meters, 15 trees are planted on each side, and one tree is planted at each of the four vertices. How many trees are planted around this site?
在一块周长为900米的正方形地四周种树，每边都种了15棵，并且四个顶点都种有一棵树．问这个场地四周共种树多少棵？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>