$R=mN=\frac{\lambda_{av}}{\Delta \lambda}=\frac{(\frac{589.592+588.995}{2})}{(589.592-588.995)}=\frac{589.7935}{0.597}=988$

$N=\frac{R}{m}$

$m=1$, $N=988$

$m=2$, $N=494$