Let f : R $ \to $ R and g : R $ \to $ R be defined as

$$f(x) = \left\{ {\matrix{ {x + a,} & {x < 0} \cr {|x - 1|,} & {x \ge 0} \cr } } \right.$ and

$g(x) = \left\{ {\matrix{ {x + 1,} & {x < 0} \cr {{{(x - 1)}^2} + b,} & {x \ge 0} \cr } } \right.$,

where a, b are non-negative real numbers. If (gof) (x) is continuous for all x $\in$$ R, then a + b is equal to ____________.