Sixth grade extracurricular activities have arranged 4 items: singing, dancing, rope skipping, table tennis, each person is required to choose one or two items to participate in. Ask at least how many students participate in extracurricular activities to ensure that at least two students choose the same project.
六年级课外活动安排了4个项目：唱歌、舞蹈、跳绳、乒乓球，规定每人从中任选一个或两个项目参加．问至少有多少个同学参加课外活动，才能保证至少有两人所选项目相同．
<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>