(R)-2-Bromobutane reacts with $\mathrm{NaN}{}_3$ in DMF by a single SN2 step. The major product and its configuration are

Azide is a good nucleophile and weak base in a polar aprotic solvent, so SN2 dominates with backside attack and inversion. In both the bromide and the azide the substituent has top priority, followed by ethyl, methyl, H, so the descriptor changes from R to S.