Hi Codeforces,

I'm currently working on a problem related to network flows. Does anyone know how to implement constraints of the following type into a system?

Edges 1 and 2 go into the vertex, and have capacity one. Edges 3 and 4 go out and have capacity one. How can one force a constraint to the effect of (edges 3 and 2 can't have a sum of flows greater than one)? 

UPD: I should note the context involves min-cost flow, so perhaps we can leverage the costs of the edges.