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This puzzle can be solved using the following concepts. Practice using these concepts and improve your skills.

Statement

 Goal

A virus is installed in one of the nodes of a network. The goal is to protect the largest possible part of the network from the virus by placing a firewall on a single node that the virus cannot traverse.
You must output the optimal location of the firewall. You cannot place your firewall on an infected node.

NOTE: The virus can spread through any link in the network. Solutions are unique, no tiebreaking is needed.
Input
Line 1: An integer, numNodes, the number of nodes in the network
Line 2: An integer, virusLocation, the starting node of the virus
Line 3: An integer, numLinks, the number of links within the network
Next numLinks lines: Two space-separated integers i and j representing the indexes of the nodes connected by the undirected link
Output
An integer firewallLocation, the index of the node where the firewall should be placed.
Constraints
5 <= numNodes <= 500
numNodes-2 < numLinks < 800
Example
Input
7
0
9
0 1
0 2
1 2
1 3
2 3
3 4
3 5
4 6
5 6
Output
3

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