using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Math;

class MainClass : Scanner
{
    static void Main()
    {
        var (N, u, v) = ReadStream<int,int,int>();
        u--;v--;
        var G = new Graph<int>(N);
        for (int i = 0; i < N-1; i++)
        {
            var r = RInts(x=>int.Parse(x)-1);
            G.Add(r[0],r[1]);
            G.Add(r[1], r[0]);
        }
        var usedt = Enumerable.Repeat(-1, N).ToArray();
        var useda = Enumerable.Repeat(-1, N).ToArray();


        var que = new Queue<(int n, int c)>();
        que.Enqueue((v, 0));
        useda[v] = 0;
        while (que.Count > 0)
        {
            var t = que.Dequeue();
            int n = t.n;
            int c = t.c;
            foreach (var i in G[n])
            {
                if (useda[i] != -1) continue;
                
                useda[i] = c + 1;
                que.Enqueue((i, c + 1));
            }
        }

        que = new Queue<(int n, int c)>();
        que.Enqueue((u, 0));
        usedt[u] = 0;
        while (que.Count > 0)
        {
            var t = que.Dequeue();
            int n = t.n;
            int c = t.c;
            foreach (var i in G[n])
            {
                if (usedt[i] != -1) continue;
                if (useda[i] < c + 1) continue;
                if(useda[i] == c + 1)
                {
                    usedt[i] = c + 1;
                    continue;
                }
                usedt[i] = c + 1;
                que.Enqueue((i, c + 1));
            }
        }
        int ans = 0;
        for (int i = 0; i < N; i++)
        {
            if (usedt[i]==-1||usedt[i] > useda[i]) continue;
            int time = usedt[i] + (useda[i] - usedt[i] - 1) / 2;
            ans = Math.Max(ans,time);
        }
        WriteLine(ans);
    }
}
class Graph<T>
{
    private List<T>[] G;
    public Graph(int size)
    {
        G = new List<T>[size];
        for (int i = 0; i < size; i++)
        {
            G[i] = new List<T>();
        }
    }

    public List<T> this[int i] => G[i];
    public void Add(int from, T to) => G[from].Add(to);
    public List<T>[] ToListArray() => G;
    public int Length => G.Length;
}
class Scanner
{
    public static int RInt() => int.Parse(Console.ReadLine());
    public static long RLong() => long.Parse(Console.ReadLine());
    public static string RString() => Console.ReadLine();
    public static double RDouble() => double.Parse(Console.ReadLine());

    public static int[] RInts() => Array.ConvertAll(Console.ReadLine().Split(), int.Parse);
    public static int[] RInts(Func<string, int> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static long[] RLongs() => Array.ConvertAll(Console.ReadLine().Split(), long.Parse);
    public static long[] RLongs(Func<string, long> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static double[] RDoubles() => Array.ConvertAll(Console.ReadLine().Split(), double.Parse);
    public static double[] RDoubles(Func<string, double> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static string[] RStrings() => Console.ReadLine().Split();
    //public static string[] RStrings(Func<string, string> func) => Console.ReadLine().Split().Select(func).ToArray();
    public static T1 ReadStream<T1>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        return r1;
    }
    public static (T1, T2) ReadStream<T1, T2>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        return (r1, r2);
    }
    public static (T1, T2, T3) ReadStream<T1, T2, T3>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        var r3 = (T3)Convert.ChangeType(r[2], typeof(T3));
        return (r1, r2, r3);
    }
    public static (T1, T2, T3, T4) ReadStream<T1, T2, T3, T4>()
    {
        var r = RStrings();
        var r1 = (T1)Convert.ChangeType(r[0], typeof(T1));
        var r2 = (T2)Convert.ChangeType(r[1], typeof(T2));
        var r3 = (T3)Convert.ChangeType(r[2], typeof(T3));
        var r4 = (T4)Convert.ChangeType(r[3], typeof(T4));
        return (r1, r2, r3, r4);
    }
}