Form a square to play a game, 15 people stand on each side, there are people at the four vertices, how many people are there in the outermost layer?
围成一个正方形做游戏，每边站15人，四个顶点都有人，最外层一共有多少人．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

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

<result>56</result>