If u\in C^2(U) and \Delta u=0 in open U\subset \mathbb{R}^n, then for every closed ball \overline{B(x,r)}\subset U one has u(x)=\frac{1}{|\partial B(x,r)|}\int_{\partial B(x,r)}u,dS = \frac{1}{|B(x,r)|}\int_{B(x,r)}u,dy.