The school organizes a movie watching. Three good friends, Xiaodong, Xiaoliang and Xiaohua, want to sit together with three tickets with consecutive numbers. Now there are 20 tickets with consecutive numbers. How many different ways do they hold?
学校组织看电影，小东、晓亮和小华三个好朋友，想拿三张连号的票坐在一起，现在有20张连号票，他们一共有多少种不同的拿法？
<gadget id="calculator">20 - 3</gadget>
<output>17</output>

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

<result>18</result>