Hello,
As a complement of what Riadh has said, you can see on the listing output(when you start running the simulation), which boundary correspond to either the number 1 or the number 2. Sometimes the boundary you think is the number 1 is not, and so with the other boundary.
In your example, having said there are two boundaries, indeed you have to set two values as Riadh and you said, although you have to make sure your south boundary is indeed the boundary number 1 (if you have a north boundary probably that one would be the number one), so check it out.
I hope this helps.
Cheers,
José Andrés.