The rotational constant of ${}^{14}\mathrm{N_2}$ is $2\,\mathrm{cm^{-1}}$. The wave number of incident radiation in a Raman spectrometer is $20487\,\mathrm{cm^{-1}}$. What is the wave number of first scattered Stokes line (in $\mathrm{cm^{-1}}$) of ${}^{14}\mathrm{N_2}$?