Limits Continuity and Differentiability

Let a function f : R $$\to$$ R be defined as f(x) = \left\{ {\matrix{ {\sin x - {e^x}} & {if} & {x \le 0} \cr {a + [ - x]} & {if} & {0 < x < 1} \cr {2x - b} & {if} & {x \ge 1} \cr } } \right. where [ x ] is the greatest integer less than or equal to x. If f is continuous on R, then (a + b) is equal to: