using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using SB = System.Text.StringBuilder;
//using System.Threading.Tasks;
//using System.Text.RegularExpressions;
//using System.Globalization;
//using System.Diagnostics;
using static System.Console;
using System.Numerics;
using static System.Math;
using pair = Pair<int, int>;

class Program
{
    static void Main()
    {
        //SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
        new Program().solve();
        Out.Flush();
    }
    readonly Scanner cin = new Scanner();
    readonly int[] dd = { 0, 1, 0, -1, 0 }; //→↓←↑
    readonly int mod = 1000000007;
    readonly int dom = 998244353;
    bool chmax<T>(ref T a, T b) where T : IComparable<T> { if (a.CompareTo(b) < 0) { a = b; return true; } return false; }
    bool chmin<T>(ref T a, T b) where T : IComparable<T> { if (b.CompareTo(a) < 0) { a = b; return true; } return false; }

    void solve()
    {
        int N = cin.nextint;
        int U = cin.nextint - 1;
        int V = cin.nextint - 1;

        var G = new Tree();
        var L = G.Input(N);

        int[] A;
        G.BFS(L, U, out A);
        int[] B;
        G.BFS(L, V, out B);

        int max = 0;
        for (int i = 0; i < N; i++)
        {
            if (A[i] <= B[i])
            {
                chmax(ref max, B[i] - 1);
            }
        }
        WriteLine(max);
        

    }

}

class Tree
{
    public List<int>[] Input(int n, int indexed = 1)
    {
        Scanner cin = new Scanner();
        var ret = new List<int>[n];
        for (int i = 0; i < ret.Length; i++) ret[i] = new List<int>();
        for (int i = 0; i < n - 1; i++)
        {
            int a = cin.nextint - indexed;
            int b = cin.nextint - indexed;
            ret[a].Add(b);
            ret[b].Add(a);
        }
        return ret;
    }
    public int BFS(List<int>[] T, int s, out int[] dist, IEnumerable<int> multi = null)
    {
        int n = T.Length;
        dist = new int[n];
        for (int i = 0; i < dist.Length; i++) dist[i] = -1;

        var Q = new Queue<int>();
        if (multi == null) { Q.Enqueue(s); dist[s] = 0; }
        else foreach (var v in multi) { Q.Enqueue(v); dist[v] = 0; }
        int id = s;

        while (Q.Count > 0)
        {
            var u = Q.Dequeue();
            foreach (var v in T[u])
            {
                if (dist[v] >= 0) continue;
                dist[v] = dist[u] + 1;
                id = v;
                Q.Enqueue(v);
            }
        }
        return id;
    }
    //public int Diameter(List<int>[] T)
    //{
    //    int[] dist;
    //    var far = BFS(T, BFS(T, 0, out dist), out dist);
    //    return dist[far];
    //}
}

static class Ex
{
    public static void join<T>(this IEnumerable<T> values, string sep = " ") => WriteLine(string.Join(sep, values));
    public static string concat<T>(this IEnumerable<T> values) => string.Concat(values);
    public static string reverse(this string s) { var t = s.ToCharArray(); Array.Reverse(t); return t.concat(); }

    public static int lower_bound<T>(this IList<T> arr, T val) where T : IComparable<T>
    {
        int low = 0, high = arr.Count;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (arr[mid].CompareTo(val) < 0) low = mid + 1;
            else high = mid;
        }
        return low;
    }
    public static int upper_bound<T>(this IList<T> arr, T val) where T : IComparable<T>
    {
        int low = 0, high = arr.Count;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (arr[mid].CompareTo(val) <= 0) low = mid + 1;
            else high = mid;
        }
        return low;
    }
}

class Pair<T, U> : IComparable<Pair<T, U>> where T : IComparable<T> where U : IComparable<U>
{
    public T f; public U s;
    public Pair(T f, U s) { this.f = f; this.s = s; }
    public int CompareTo(Pair<T, U> a) => f.CompareTo(a.f) != 0 ? f.CompareTo(a.f) : s.CompareTo(a.s);
    public override string ToString() => $"{f} {s}";
}

class Scanner
{
    string[] s; int i;
    readonly char[] cs = new char[] { ' ' };
    public Scanner() { s = new string[0]; i = 0; }
    public string[] scan => ReadLine().Split();
    public int[] scanint => Array.ConvertAll(scan, int.Parse);
    public long[] scanlong => Array.ConvertAll(scan, long.Parse);
    public double[] scandouble => Array.ConvertAll(scan, double.Parse);
    public string next
    {
        get
        {
            if (i < s.Length) return s[i++];
            string st = ReadLine();
            while (st == "") st = ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            i = 0;
            return next;
        }
    }
    public int nextint => int.Parse(next);
    public long nextlong => long.Parse(next);
    public double nextdouble => double.Parse(next);
}


Measured memory_bytes: 30348