There are 5 red balls, 5 yellow balls and 5 blue balls of the same size in the box. If you want to draw out the balls, there must be two of the same color. The minimum number of balls you need to touch.
盒子里有同样大小的红球、黄球和蓝球各5个，要想摸出的球一定有两个同色的，最少要摸多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>