Problem:
Score : 600 points

Problem Statement

We have a tree with N vertices. The i-th edge connects Vertex A_i and B_i bidirectionally.

Takahashi is standing at Vertex u, and Aoki is standing at Vertex v.

Now, they will play a game of tag as follows:

1. If Takahashi and Aoki are standing at the same vertex, the game ends. Otherwise, Takahashi moves to a vertex of his choice that is adjacent to his current vertex.

2. If Takahashi and Aoki are standing at the same vertex, the game ends. Otherwise, Aoki moves to a vertex of his choice that is adjacent to his current vertex.

3. Go back to step 1.

Takahashi performs his moves so that the game ends as late as possible, while Aoki performs his moves so that the game ends as early as possible.

Find the number of moves Aoki will perform before the end of the game if both Takahashi and Aoki know each other's position and strategy.

It can be proved that the game is bound to end.

Constraints

2 \leq N \leq 10^5

1 \leq u,v \leq N

u \neq v

1 \leq A_i,B_i \leq N

The given graph is a tree.

Input

Input is given from Standard Input in the following format:

N u v
A_1 B_1
:
A_{N-1} B_{N-1}

Output

Print the number of moves Aoki will perform before the end of the game.

Sample Input 1

5 4 1
1 2
2 3
3 4
3 5

Sample Output 1

2

If both players play optimally, the game will progress as follows:

Takahashi moves to Vertex 3.

Aoki moves to Vertex 2.

Takahashi moves to Vertex 5.

Aoki moves to Vertex 3.

Takahashi moves to Vertex 3.

Here, Aoki performs two moves.

Note that, in each move, it is prohibited to stay at the current vertex.

Sample Input 2

5 4 5
1 2
1 3
1 4
1 5

Sample Output 2

1

Sample Input 3

2 1 2
1 2

Sample Output 3

0

Sample Input 4

9 6 1
1 2
2 3
3 4
4 5
5 6
4 7
7 8
8 9

Sample Output 4

5

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main{
    
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int n = Integer.parseInt(sc.next());
        int a = Integer.parseInt(sc.next())-1;
        int b = Integer.parseInt(sc.next())-1;
        
        Dijkstra dij = new Dijkstra(n);
        
        for(int i=0; i<n-1; i++){
            int u = Integer.parseInt(sc.next())-1;
            int v = Integer.parseInt(sc.next())-1;
            dij.addEdge(u,v,1);
        }
        
        long[] adist = dij.dijkstra(a);
        long[] bdist = dij.dijkstra(b);
        
        long firstDist = adist[b];
        
        long max = 0;
        for(int i=0; i<n; i++){
            if(adist[i] < bdist[i]){
                max = Math.max(max, bdist[i]);
            }
            //System.out.println("a " + adist[i] + " ; b " + bdist[i]);
        }
        
        System.out.println(Math.max(max-1, firstDist/2));
        //System.out.println(firstDist/2);
        //System.out.println(max-1);
    }
}

class Dijkstra{
    
    public final long INF = (long)1e18;
    private int n;
    private List<Edge>[] edge;
    
    public Dijkstra(int n){
        this.n = n;
        edge = new List[n];
        for(int i=0; i<n; i++){
            edge[i] = new ArrayList<>();
        }
    }
    
    public void addEdge(int from, int to, int cost){
        edge[from].add(new Edge(from, to, cost));
        edge[to].add(new Edge(to, from, cost));
    }
    
    public long[] dijkstra(int from){
        long[] dist = new long[n];
        Arrays.fill(dist, INF);
        dist[from] = 0;
        
        PriorityQueue<Vertex> q = new PriorityQueue<>();
        q.add(new Vertex(from, 0));
        while(q.size() != 0){
            int u = q.poll().ver;
            
            for(Edge ne : edge[u]){
                int v = ne.to;
                long weight = ne.cost;
                if(dist[v] > dist[u]+weight){
                    dist[v] = dist[u]+weight;
                    q.add(new Vertex(v, dist[v]));
                }
            }
        }
        
        return dist;
    }
}

class Edge implements Comparable<Edge>{
    public int from = 0;
    public int to = 0;
    public int cost = 0;
    
    public Edge(int from, int to, int cost) {
        this.from = from;
        this.to = to;
        this.cost = cost;
    }
    
    @Override
    public int compareTo(Edge o) {
        return this.cost - o.cost;
    }
}

class Vertex implements Comparable<Vertex>{
    public int ver = 0;
    public long cost = 0;
    
    public Vertex(int ver, long cost) {
        this.ver = ver;
        this.cost = cost;
    }
    
    @Override
    public int compareTo(Vertex o) {
        long tmp = this.cost - o.cost;
        if(tmp == 0) return 0;
        return (int)(tmp/Math.abs(tmp));
    }
}