If you know the divergence theorem it is simply 2*V = 4pi

Even though this is not a closed surface the flux through the right and left faces is 0 (j component of F is 0) you can close up the cylinder and use the divergence theorem

If Not

The equation of the cylinder is then x^2 +z^2 = 1 -1< y <1

You'll have to break this up into the upper half and lower half over the rectangle -1 < x < 1 and -1 < y <1

For the upper half N = x/z i + k For the lower half N = -x/z i - k

You'll get 2pi in both cases