The children are doing group exercises and they are arranged in a hollow phalanx with 2 layers. There are 9 people on each side of the outer layer. How many children are doing group exercises?
小朋友跳集体操，排成一个2层的空心方阵，外层每边有9人，跳集体操的小朋友共有多少人．
<gadget id="calculator">9 - 2</gadget>
<output>7</output>

<gadget id="calculator">7 * 2 * 4</gadget>
<output>56</output>

<result>56</result>