combinatorics - Prove by induction that $sumlimits_{k=m}^{ n}{nchoose k}{kchoose m}={nchoose m}2^{n-m}$.

Prove by induction that $\displaystyle\sum\limits_{k=m}^{\ n}{n\choose k}{k\choose m}={n\choose m}2^{n-m}$.

I can't figure out what is the base case. Could someone show the steps?