Derivation of Magnetic field Boundary Conditions

One of Maxwell's equations state that for a magnetic field B

divB=0
Integrate over volume of interest and using Gauss's law and the equation becomes
INTsc B.ds= 0
where INTsc refers to an area intergration over a closed surface. If this imaginary area incorporates two materials equally, and noting that the magnetic flux ψ is given by
ψ=INTs B.ds
then we can conclude that
ψ12
where ψ12 are the magnetic fluxes in material 1 and material 2 respectively

        ..........................................
                                                 
        .              ψ1                            material 1
                                                  
        --------------------------------
                                           
                         ψ2                 
                                               material 2
        ..........................................