20 teachers went to the show and bought 20 tickets with consecutive numbers. Among them, Teacher Li, Teacher Zhang, and Teacher Wang wanted to sit together. They wanted to take three tickets with consecutive numbers. How many different ways are there in total?
20个教师去看演出，买了20张连号的票，其中李老师、张老师、王老师很想坐在一起，他们要拿三张连号的票，一共有多少不同的拿法．
<gadget id="calculator">20 - 3</gadget>
<output>17</output>

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

<result>18</result>