20 children stand in a row and choose three adjacent children to perform. How many different performance methods are there?
20名小朋友站成一排，选相邻的三个小朋友表演节目，有多少不同的表演方法．
<gadget id="calculator">20 - 3</gadget>
<output>17</output>

<gadget id="calculator">17 + 1</gadget>
<output>18</output>

<result>18</result>