Markers on a Tree (Easy)

**This is the easy version of the problem. Here, you only need to find $f(S)$.** For a tree $T$ consisting of $N$ labelled nodes, labelled $1$ through $N$; consider having $2$ markers, marker $A$ initially located at node $1$ and marker $B$ located at node $N$. In one move, you can move **exactly** one of the $2$ markers from it's current node to an adjacent node. A node is considered visited if