Wave Optics

In Young's double slit experiment, two wavelengths $\lambda_1=780 \mathrm{~nm}$ and $\lambda_2=520 \mathrm{~nm}$ are used to obtain interference fringes. If the $n^{\mathrm{th}}$ bright band due to $\lambda_1$ coincides with $(n+1)^{\text {th }}$ bright band due to $\lambda_2$, then the value of $n$ is