Bipartite Graphs and Matchings · Easy problem

Six guard posts $1,2,\dots,6$ are arranged in a ring, each able to monitor the two links to its neighbors in the cycle $1\text{-}2\text{-}3\text{-}4\text{-}5\text{-}6\text{-}1$. What is the minimum number of posts that must be staffed so every link in the ring is monitored by at least one staffed post?