There are 10 balls in each of the four colors of red, yellow, blue, and green. If two of the balls to be taken out are of the same color, how many balls must be taken out at least.
有红、黄、蓝、绿四种颜色的球各10个，要想取出的球有两个是同色的，至少要取出多少个球．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<result>5</result>