There are 6 white balls, 6 blue balls, and 6 red balls of the same size in the box. If you want to get 2 balls of the same color, how many balls should you touch at least?
盒子里有同样大小的白球、蓝球、红球各6个，要想摸出的球一定有2个同色的，最少要摸多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>