using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using static System.Math;
using static System.Array;
using static AtCoder.Tool;
namespace AtCoder
{
    class AC
    {
        const int MOD = 1000000007;
        const int INF = int.MaxValue / 2;
        const long SINF = long.MaxValue / 2;
        const double EPS = 1e-8;
        static readonly int[] dx = { 0, 1, 0, -1 };
        static readonly int[] dy = { 1, 0, -1, 0 };
        static List<List<int>> G = new List<List<int>>();
        //static List<List<Edge>>G = new List<List<Edge>>();
        //static List<Edge> E = new List<Edge>();
        
        static void Main(string[] args)
        {
            //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            //Console.SetOut(sw);
            var cin = new Scanner();

            var input = cin.ReadSplit();
            int N = input[0];
            int u = input[1];u--;
            int v = input[2];v--;

            for (var i = 0; i < N; i++) G.Add(new List<int>());
            for(var i = 0; i < N - 1; i++)
            {
                var p = cin.ReadSplit();
                p[0]--;
                p[1]--;
                G[p[0]].Add(p[1]);
                G[p[1]].Add(p[0]);
            }
            var dfs = new Stack<Tuple<int, int, long>>();
            dfs.Push(new Tuple<int, int, long>(v, -1, 0));
            var dv = new long[N];
            while (dfs.Any())
            {
                var cur = dfs.Pop();
                int cv = cur.Item1;
                int prev = cur.Item2;
                long dist = cur.Item3;
                dv[cv] = dist;

                foreach(var nx in G[cv])
                {
                    if (prev == nx) { continue; }
                    dfs.Push(new Tuple<int, int, long>(nx, cv, dist + 1));
                }
            }

            var du = new long[N];
            dfs.Push(new Tuple<int, int, long>(u, -1, 0));
            while (dfs.Any())
            {
                var cur = dfs.Pop();
                int cv = cur.Item1;
                int prev = cur.Item2;
                long dist = cur.Item3;
                du[cv] = dist;

                foreach (var nx in G[cv])
                {
                    if (prev == nx) { continue; }
                    dfs.Push(new Tuple<int, int, long>(nx, cv, dist + 1));
                }
            }

            long ans = 0;
            for(var i = 0; i < N; i++)
            {
                if (du[i] < dv[i]) { ans = Max(ans, dv[i] - 1); }
            }
            Console.WriteLine(ans);


            //Console.Out.Flush();
        }
        struct Edge
        {
            public int from;

            public int to;
            public long dist;
            public Edge(int t, long c)
            {
                from = -1;
                to = t;
                dist = c;
            }
            public Edge(int f, int t, long c)
            {
                from = f;
                to = t;
                dist = c;
            }

        }
    }
    public class Scanner
    {
        public int[] ReadSplit()
        {
            return ConvertAll(Console.ReadLine().Split(), int.Parse);
        }
        public long[] ReadSplitL()
        {
            return ConvertAll(Console.ReadLine().Split(), long.Parse);
        }
        public double[] ReadSplitD()
        {
            return ConvertAll(Console.ReadLine().Split(), double.Parse);
        }
    }
    public static class Tool
    {
        static public void Initialize<T>(ref T[] array, T initialvalue)
        {
            for (var i = 0; i < array.Length; i++)
            {
                array[i] = initialvalue;
            }
        }
        static public void Swap<T>(ref T a, ref T b)
        {
            T keep = a;
            a = b;
            b = keep;
        }

        static public void Display<T>(T[,] array2d, int n, int m)
        {
            for (var i = 0; i < n; i++)
            {
                for (var j = 0; j < m; j++)
                {
                    Console.Write($"{array2d[i, j]} ");
                }
                Console.WriteLine();
            }
        }

    }
}


Measured memory_bytes: 35672