Let the functions f : (−1, 1) → ℝ and g : (−1, 1) → (−1, 1) be defined by f(x) = |2x − 1| + |2x + 1| and g(x) = x − [x], where [x] denotes the greatest integer less than or equal to x. Let f ∘ g : (−1, 1) → ℝ be the composite function defined by (f ∘ g)(x) = f(g(x)). Suppose c is the number of points in the interval (−1, 1) at which f ∘ g is NOT continuous, and suppose d is the number of points in the interval (−1, 1) at which f ∘ g is NOT differentiable. Then the value of c + d is _____.