I need to solve 2D Poisson equation, using multigrid (v_cycle) on the unit square with boundary Dirichlet conditions on three sides and Neumann on one side.
The problem is, that I don't know what to do with Neumann condition?

If all four conditions were Dirichlet, then it is clear that only
interior points are unknown. But what happens when it is Neumann condition on one side?

Thank you!