A circuit is given with the following parameters
$I_1 = 2 ~{\rm A}$
$R_2 = 5 ~\Omega$
$R_3 = 20 ~\Omega$
$U_3 = 1 ~{\rm V}$
$R_4 = 10 ~\Omega$
$U_4 = 3 ~{\rm V}$

Determine the open circuit voltage between A and B using the principle of superposition.