There are 10 red balls, 10 green balls, 10 black balls and 10 white balls of the same size in the cloth bag, at least how many balls must be drawn to ensure that 2 balls of the same color are taken out.
布袋里有同样重量，同样大小的红球、绿球、黑球和白球各10个，至少要摸出多少个球，才能保证有2个同色的球被取出．
<gadget id="calculator">1 * 4</gadget>
<output>4</output>

<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<result>5</result>