Maximum Flow in a Network Let $N = (V, E)$ be a network in which the capacity of each edge is either $12$ or $18$.

Prove or disprove:
The value of a maximum flow for $N$ can’t be $56$.

I'm trying to figure out how to definitely prove this.
I think that this is not possible because of no combination of $12X + 18Y$ (where $X$ and $Y$ are integers) will ever $= 56$. Is there a better way of saying this? 
Am I right to say that an integer solution to $12X + 18Y = 56$ is what the Fold-Fulkerson algorithm implies?
By the max flow min cut theorem, the maximum value of a flow equals the minimum capacity of a cut. The capacity of a cut is of the form 12x+18y, which can’t equal 56 because 56 is not a multiple of 6.