There are 7 red balls, 7 yellow balls, and 7 basketballs of the same size in the box. If you want to draw out the balls, there must be 2 of the same color, at least how many balls you need to touch.
盒子里有同样大小的红球、黄球、篮球各7个，要想摸出的球一定有2个同色的，至少要摸多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>