Algorithms – Maximum flow in a network

Let N = (V, E) be a network in which the capacity of each edge is 12 or 18.
Test or disprove:
The value of a maximum flow for N can not be 56.

I'm trying to figure out how to definitely try this.
I think this is not possible because a combination of 12X + 18Y (where X and Y are integers) will never be = 56. Is there a better way to say this?
Am I right in saying that y (X, Y) the whole solution for 12X + 18Y = 56 is what the Fold-Fulkerson algorithm implies?