Four jugs of the same size and weight are filled with milk of different weights. If two of the cans of milk are weighed randomly, the weights of 6 cans are 13, 14, 15, 16, 17, and 18 kilograms. It is known that the sum of the weights of the four empty tanks and the sum of the weights of the clean milk (calculated in kilograms) are two different prime numbers, and the weight of the milk in each tank is not less than 1 kg. Q: How much is the total weight of the net milk in the two lightest cans?
四个大小和重量都相同的罐，分别盛着不同重量的牛奶．如果任意把其中两罐牛奶称重，可称出6个分别是13、14、15、16、17、18千克的重量．已知四个空罐的重量总和及净牛奶的重量总和(以千克计算)，分别是两个不同的质数，且每个罐内的牛奶重量不少于1千克．问：最轻的两罐内净牛奶共重多少千克？
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) * 2</gadget>
<output>1</output>

<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<result>12</result>