Mary has 6 cards, each of which has a positive integer written on it. After she selected 3 cards, she counted their total number. She chooses another 3 cards, and then calculates the total number on these 3 cards. She made 20 possible 3-card choices, then counted, and found that 10 totaled 16 and the other 10 totaled 18. What is the smallest number among these cards?
玛丽有6张卡片，每张卡片上都写有一个正整数．她选取了3张卡片后，算出了它们的总数．她又选另外3张卡片，再算出这3张卡片上的总数．她进行了可能的20种3张卡片的选择，然后计算，发现有10种总数等于16，另10种总数等于18．请问这些卡片中最小的数是多少？
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<result>4</result>