To welcome Children's Day, the school held a group gymnastics performance. The fourth grade selects some students to participate and arranges them in a square array. There are 15 people standing on each side of the outermost layer. How many students are there in the outermost layer?
为迎接六一儿童节，学校举行团体体操表演．四年级选出部分学生参加，排成一个正方形的方阵，最外层每边站15人，最外层一共有多少名学生？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>