A chemical reaction produces a substance $S$ at a rate proportional to the amount of reactant $R$ remaining. If $R(0) = R_0$ and the reaction rate is $\frac{dR}{dt} = -kR$, what is the expression for the amount of substance $S(t)$ produced if $S(0) = 0$ and $S(t) + R(t) = R_0$?