18 students stand in line, numbered from 1 to 18. The physical education teacher asked students from numbers 11 to 15 to stand up. How many people are left in the team?
18个同学站队，编号从1到18．体育老师让11到15号的同学站出来，队伍里还剩多少人？
<gadget id="calculator">15 - 11</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">18 - 5</gadget>
<output>13</output>

<result>13</result>