Huanhuan set up a five-layer hollow square array with chess pieces. There are 18 pieces on each side of the outermost layer. How many chess pieces did Huanhuan use to form this square array?
欢欢用棋子摆了一个五层空心方阵，最外层每边有18颗棋子，欢欢摆这个方阵共用了多少颗棋子？
<gadget id="calculator">18 - 5</gadget>
<output>13</output>

<gadget id="calculator">13 * 5 * 4</gadget>
<output>260</output>

<result>260</result>