Two wavelengths $\lambda_1$ and $\lambda_2$ are used in Young's double slit experiment $\lambda_1=450 \mathrm{~nm}$ and $\lambda_2=650 \mathrm{~nm}$. The minimum order of fringe produced by $\lambda_2$ which overlaps with the fringe produced by $\lambda_1$ is n . The value of $n$ is