Neumann boundary conditions

[ma-sadeghi]

I'm trying to solve the Laplace PDE in 2D with the following boundary conditions:

x = 0 ; u = 1
x = 1 ; u = 0
y = 0 ; du/dy = 0
y = 1 ; du/dy = 0

How can I enforce the du/dy=0 ones? Thanks!