## Editorial for COCI '20 Contest 2 #2 Odašiljači

Remember to use this editorial

**only**when stuck, and**not to copy-paste code from it**. Please be respectful to the problem author and editorialist.**Submitting an official solution before solving the problem yourself is a bannable offence.**First, let's discuss how to check for some fixed radius if all antennas can communicate. Two antennas can communicate directly if their distance is at most . We can build a graph over antennas, and add edges for all pairs of antennas that are closer than . Then, we need to check if the graph is connected, which can be done for example with DFS.

The optimal , that is the minimal for which the constructed graph is connected, can be found using binary search. For the upper bound, we can put for example . The complexity is .

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