There are football, table tennis, badminton, and basketball in the sports room of the school. Each student chooses two of them at random. How many students are needed to ensure that two people borrow the same ball?
学校体育室有足球、乒乓球、羽毛球、篮球四种球，每个学生从中任意选择2个，那么至少要几个学生才能保证一定有2人所借的球是一样的？
<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<gadget id="calculator">10 + 1</gadget>
<output>11</output>

<result>11</result>