Insert colored flags around a square field, and each side must insert 15 sides, at least how many sides must be inserted.
在一块正方形场地四周插上彩旗，每边都要插15面，至少要插多少面．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>