Put 6 balls of red, yellow and blue in each color and put them in a cloth bag after mixing. How many balls should be drawn out at a time to ensure that there are two balls of the same color?
红、黄、蓝三种颜色的球各6个，混合后放在一个布袋里，一次至少摸出几只，才能保证有两只是同色的？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>