There are some small balls in a bag. They are only different in color, and the others are exactly the same, among which 10 are red, 8 are yellow, and 2 are blue. So how many balls should be taken at a time to ensure that 4 balls have the same color?
一个布袋中有一些小球．它们仅是颜色不同，其它完全一样，其中红色的10个，黄色的8个，蓝色的2个．那么一次最少取多少个球，才能保证有4个球的颜色相同？
<gadget id="calculator">2 + 3 + 4</gadget>
<output>9</output>

<result>9</result>