For a chemical reaction $A \to B$, the rate of the reaction is $2 \times 10^{-3}$ mol dm$^{-3}$ s$^{-1}$, when the initial concentration is 0.05 mol dm$^{-3}$. The rate of the same reaction is $1.6 \times 10^{-2}$ mol dm$^{-3}$ s$^{-1}$ when the initial concentration is 0.1 mol dm$^{-3}$. The order of the reaction is