There are 6 red, yellow, blue, and white handkerchiefs each. Mix them in a cloth bag and touch at least how many handkerchiefs at a time to ensure that there are 4 handkerchiefs of different colors.
有红、黄、蓝、白色的手帕各6条，混合在一个布袋里一次至少摸多少条才能保证有4条不同色的手帕．
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 + 1</gadget>
<output>19</output>

<result>19</result>