A box contains 10 red balls and 8 white balls of the same size. At least how many more balls must be drawn to ensure that 4 balls are of the same color.
一个盒子里有同样大小的红球10个，白球8个．至少要摸出多多少个球，才能保证有4个球颜色是相同的．
<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>