Reach Anywhere

You are given a simple undirected graph with $N$ vertices and $M$ edges. Find the smallest non-negative integer $K$ such that for *every* vertex $u$ ($1 \leq u \leq N$), there exists a [*walk*](https://mathworld.wolfram.com/Walk.html) of length **exactly** $K$ from $1$ to $u$. If no such integer exists, print $-1$ instead. **Notes:** - It is allowed to repeat both vertices and edges in a walk.