The oldest student in a certain school is 13 years old, and the youngest is 6 years old. At least how many students need to be selected, and two students with the same age can be found.
某校学生的年龄最大13岁，最小6岁，至少需从多少个学生中挑选，就一定能找到两个学生岁数相同．
<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>