The minimum age of primary school students is 6 years old, and the maximum age is 13 years old. How many students can be selected from the school to ensure that there are two students of the same age.
小学生最小年龄为6岁，最大年龄为13岁，从学校任选多少名同学能保证有两位同学的年龄相同．
<gadget id="calculator">13 - 6</gadget>
<output>7</output>

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

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

<result>9</result>