Let f: R → R be a differentiable function that satisfies the relation f(x+y)=f(x)+f(y)-1, ∀ x, y ∈ R. If f^′(0)=2, then |f(-2)| is equal to ____.

Let $\displaystyle f: \mathbb{R} \rightarrow \mathbb{R}$ be a differentiable function that satisfies the relation $\displaystyle f(x+y)=f(x)+f(y)-1, \forall x, y \in \mathbb{R}$. If $\displaystyle f^{\prime}(0)=2$, then $\displaystyle |f(-2)|$ is equal to $\displaystyle \_\_\_\_$.