There are 5 white balls, 5 red balls, 5 yellow balls, and 5 blue balls of the same size in the box, and at least how many balls can be drawn to ensure that there must be two balls of the same color.
盒子里有同样大小的白球、红球、黄球、蓝球各5个，至少摸出多少个球，才可以保证一定有两个颜色相同的球．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<result>5</result>