There are three number cards, which are written 1, 2, and 3 respectively. Students choose two cards at random to form a two-digit number. At least how many students are required to participate in order to ensure that two of them have the same double digits in their cards?
有三张数字卡片，分别写着1，2，3，同学们任意选取两张组成一个两位数．至少要几名同学参与才能保证其中有两人的卡片所组成的两位数相同？
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>