6. The students in the second class participate in social practice activities and are required to participate in one or two of the four places A, B, C, D. At least how many students must visit to ensure that two people visit exactly the same place?
六、二班的同学参加社会实践活动，要求参加A，B，C，D，4个地方中的一处或两处，至少有几名同学参观才能保证有两人参观的地方完全相同？
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

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

<result>11</result>