There are 10 red, yellow and blue balls in the box. If you close your eyes and draw out 2 balls of the same color, at least how many balls must be drawn out.
箱子里有红，黄，蓝三种颜色的小球各10个，如果让你闭上眼睛摸出的球一定有2个同色，至少要摸出多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>