using System;
using System.Collections.Generic;

public class Program
{
    private int N;
    private int U, V;
    private List<int>[] E;
    int[] Dist;

    public void Solve()
    {
        var sc = new Scanner();
        N = sc.NextInt();
        U = sc.NextInt() - 1;
        V = sc.NextInt() - 1;
        E = new List<int>[N];
        for (int i = 0; i < N; i++)
        {
            E[i] = new List<int>();
        }

        for (int i = 0; i < N - 1; i++)
        {
            int a = sc.NextInt() - 1;
            int b = sc.NextInt() - 1;
            E[a].Add(b);
            E[b].Add(a);
        }

        // Tが部分木の根にいてAがその親にいる
        // Aは部分木の葉に移動するしかない

        // T Aの方に行く -> 一番遠い葉に行く
        // A Tを追いかける V-葉の親 が答え
        Dist = new int[N];
        Search(V, 0, -1);

        int parent = U;
        int root = U;
        for (int i = 0; i < (Dist[U] + 1) / 2; i++)
        {
            foreach (var j in E[root])
            {
                if (Dist[j] == Dist[root] - 1)
                {
                    parent = root;
                    root = j;
                    break;
                }
            }
        }

        //Console.WriteLine($"{parent} {root}");
        //Console.WriteLine($"{Dist[2]} {Dist[3]} {Dist[4]}");
        Console.WriteLine(SearchB(parent,root)-1);
    }

    int SearchB(int i, int parent)
    {
        int ans = Dist[i];
        foreach (var j in E[i])
        {
            if (j == parent) continue;
            ans = Math.Max(ans, SearchB(j, i));
        }
        return ans;
    }

    void Search(int i, int dist, int parent)
    {
        Dist[i] = dist;
        foreach (int j in E[i])
        {
            if (parent == j) continue;
            Search(j, dist + 1, i);
        }
    }

    public static void Main(string[] args)
    {
        new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _pos = 0;
        _line = new string[0];
    }

    const char Separator = ' ';
    private int _pos;
    private string[] _line;

    #region スペース区切りで取得

    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = Console.ReadLine().Split(Separator);
            _pos = 0;
        }

        return _line[_pos++];
    }

    public int NextInt()
    {
        return int.Parse(Next());
    }

    public long NextLong()
    {
        return long.Parse(Next());
    }

    public double NextDouble()
    {
        return double.Parse(Next());
    }

    #endregion

    #region 型変換

    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }

    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }

    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }

    #endregion

    #region 配列取得

    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = Console.ReadLine().Split(Separator);

        _pos = _line.Length;
        return _line;
    }

    public int[] IntArray()
    {
        return ToIntArray(Array());
    }

    public long[] LongArray()
    {
        return ToLongArray(Array());
    }

    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }

    #endregion
}

Measured memory_bytes: 35796