Black and White

You are given an **simple, undirected, connected** graph with $\textbf{n}$ nodes (numbered from $1$ to $n$) and $\textbf{m}$ edges, with each node coloured either black or white. There are a total $\textbf{b}$ black nodes. All edges in the graph have weight = 1. A node is called $\textbf{magical}$ if it is a $\textbf{white node}$ and $\textbf{each black node}$ is at atmost $\textbf{k}$ distance f