The youngest student in the school is 6 years old, and the oldest is 13 years old. How many students are randomly selected from the students, and it is guaranteed that there are three students of the same age?
学校里最小的学生6岁，最大的13岁，从学生中任意选取多少位，一定能保证有三名学生同龄？
<gadget id="calculator">13 - 6</gadget>
<output>7</output>

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<gadget id="calculator">8 * 2</gadget>
<output>16</output>

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

<result>17</result>