$(R)$-2-Bromobutane is treated with $\mathrm{NaCN}$ in DMSO and the reaction proceeds entirely by SN2. The product is

Inversion occurs at the stereocentre. The priorities at the carbon are $\mathrm{Br}>\mathrm{C}{}_2\mathrm{H}{}_5>\mathrm{CH}{}_3>\mathrm{H}$ in the substrate and $\mathrm{CN}>\mathrm{C}{}_2\mathrm{H}{}_5>\mathrm{CH}{}_3>\mathrm{H}$ in the product (the cyano carbon has substituents (N,N,N), higher than (C,H,H)). The order is the same, so the descriptor changes from $R$ to $S$. Cyanide in DMSO attacks through carbon, so the product is a nitrile: $(S)$-2-methylbutanenitrile.