For any prime $p$, $λ$-constacyclic codes of length $p^s$ over ${\cal R}=\mathbb{F}_{p^m} + u\mathbb{F}_{p^m}$ are precisely the ideals of the local ring ${\cal R}_λ=\frac{{\cal R}[x]}{\left\langle x^{p^s}-λ\right\rangle}$, where $u^2=0$. In this paper, we first investigate the Hamming distances of cyclic codes of length $p^s$ over ${\cal R}$. The minimum Hamming distances of all cyclic codes of length $p^s$ over ${\cal R}$ are determined. Moreover, an isometry between cyclic and $α$-constacyclic codes of length $p^s$ over ${\cal R}$ is established, where $α$ is a nonzero element of $\mathbb{F}_{p^m}$, which carries over the results regarding cyclic codes corresponding to $α$-constacyclic codes of length $p^s$ over ${\cal R}$.