Can you give an example of some Boolean algebra $\mathcal A$ over $\mathbb N$ and a non-zero finitely additive measure $\mu$ on this algebra, which has a countably additive extension to the $\sigma$-algebra generated by this algebra, and moreover, such that when shifting any set $A ∈ \mathcal A$ by an integer $n$ (calling the resulting set $A + n$), the following equation is fulfilled: $A + n ∈ \mathcal{A}$, $\mu (A + n) = \mu (A)$?