A carton contains 9 red balls, 8 green balls and 6 yellow balls, which are identical except for their colour. How many small balls must be drawn at a time to ensure that there are two red balls and two yellow balls among the small balls drawn.
一个纸箱里装有9个红球、8个绿球和6个黄球，它们除颜色外完全相同．一次最少摸出多少个小球才能保证摸出的小球中有两个红球和两个黄球．
<gadget id="calculator">8 + 9 + 2</gadget>
<output>19</output>

<result>19</result>