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