Let $\mathcal{M} = (\mathbb{Z}, S)$ be a structure consisting of the set of integers $\mathbb{Z}$ and the successor relation $S(x, y) \iff y = x + 1$. Which of the following binary relations $R(x, y)$ on $\mathbb{Z}$ is definable by a first-order formula in the language containing only $S$ and equality?