There are 3 red balls, 5 yellow balls, and 7 blue balls of the same size in an opaque box. Feel free to draw some balls. If you want the balls to be drawn, there must be three colors. How many balls must be drawn at least?
在一个不透明的盒子里有同样大小的红球3个，黄球5个，蓝球7个．随意摸出一些球，要想摸出的球一定三种颜色，至少要摸出几个球？
<gadget id="calculator">7 + 5 + 1</gadget>
<output>13</output>

<result>13</result>