In Young's double-slit experiment, two wavelengths ${\lambda }_1=780 \mathrm{n}\mathrm{m}$ and ${\lambda }_2=520 \mathrm{n}\mathrm{m}$ are used to obtain interference fringes. If the $n^{th}$ bright band due to ${\lambda }_1$ coincides with ${\left(n+1\right)}^{th}$ bright band due to ${\lambda }_2$, then the value of $n$ is