20 children stand in a row. If you want to choose 4 adjacent children to perform, how many different ways are there?
20个小朋友站成一排．如果要选相邻的4个小朋友表演节目，有多少种不同的选法？
<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">16 + 1</gadget>
<output>17</output>

<result>17</result>