Among the primary school students in a certain school, the youngest is 6 years old, and the oldest is 13 years old. If you select at least a few students from this school, it is guaranteed that there must be three students of the same age?
某校的小学生中，年龄最小的6岁，最大的13岁，从这个学校中至少选几个学生，就能保证其中一定有三个学生的年龄相同？
<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>