The oldest student in a certain elementary school is 13 years old, and the youngest is 6 years old. At least how many students must be selected from among them, so that two of the selected students must be of the same age.
某小学的学生年龄最大的为13岁，最小的为6岁，至少要从中选多少名学生，就一定能使挑出的学生中有两名学生的岁数相同．
<gadget id="calculator">13 - 6</gadget>
<output>7</output>

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

<gadget id="calculator">8 + 1</gadget>
<output>9</output>

<result>9</result>