\int_{S+S_1'+S_0}(\varphi\frac{\partial}{\partial n}\frac{1}{r'}-\frac{1}{r'}\frac{\partial \varphi}{\partial n})dS=\int_{S}(\varphi\frac{\partial}{\partial n}\frac{1}{r'}-\frac{1}{r'}\frac{\partial \varphi}{\partial n})dS+\int_{S_1'}(\varphi\frac{\partial}{\partial n}\frac{1}{r'}-\frac{1}{r'}\frac{\partial \varphi}{\partial n})dS+\int_{S_0}(\varphi\frac{\partial}{\partial n}\frac{1}{r'}-\frac{1}{r'}\frac{\partial \varphi}{\partial n})dS