On the edge of a square pool, put red flags, one on each vertex, 15 on each side, how many red flags are there in total?
在一个正方形的水池边，插红旗，每个顶点上插一面，每边有15面，一共有多少面红旗．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>