There is a square playground, 15 trees should be planted on each side, and one tree should be planted on each of the four corners. How many trees should be planted around the playground?
有一个正方形的操场，每边都要种15棵树，而且四个角上都要种一棵，这个操场的四周一共要种多少棵树？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>