392. How Far Is the Furthest
Print the distance from vertex 1 to the vertex furthest from it, counted in
edges. Print -1 if any vertex cannot be reached from vertex 1 at all.
This number is called the eccentricity of vertex 1.
The search is the one from problem 364, run once. What this problem adds is that you must notice the unreachable case rather than quietly reporting the furthest thing you happened to find. A graph in two pieces has a largest distance inside vertex 1's own piece, and reporting it would be wrong.
Constraints - `1 ≤ n ≤ 100000` - `0 ≤ m ≤ 200000` - There are no self-loops and no repeated edges.
Input
The first line contains two integers n and m, the number of vertices and
edges. Each of the next m lines contains two integers u and v, an
undirected edge between those vertices. Vertices are numbered 1 to n.
Output
Print one integer, the largest distance from vertex 1, or -1 if some vertex is
unreachable.
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.