There are red, yellow, and blue balls, if you want to draw 2 balls of the same color, at least how many balls must be drawn.
有红、黄、蓝三种颜色的球，如果要保证摸出2个相同颜色的球，至少要摸出多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>