using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Text;
using System.Linq;

using E = System.Linq.Enumerable;

internal partial class Solver {
    public void Run() {
        var n = ni();
        var u = ni() - 1;
        var v = ni() - 1;
        var adj = Enumerable.Range(0, n).Select(_ => new List<int>()).ToArray();
        for (int i = 0; i < n - 1; i++) {
            var a = ni() - 1;
            var b = ni() - 1;
            adj[a].Add(b);
            adj[b].Add(a);
        }
        var parent = E.Repeat(-1, n).ToArray();
        var depth = E.Repeat(-1, n).ToArray();

        var queue = new Queue<int>();
        queue.Enqueue(v);
        queue.Enqueue(-1);
        queue.Enqueue(0);
        var perm = new List<int>();
        while (queue.Count > 0) {
            var current = queue.Dequeue();
            var pa = queue.Dequeue();
            var dep = queue.Dequeue();
            if (depth[current] != -1) continue;
            perm.Add(current);
            depth[current] = dep;
            parent[current] = pa;

            foreach (var next in adj[current]) {
                if (next != pa) {
                    queue.Enqueue(next);
                    queue.Enqueue(current);
                    queue.Enqueue(dep + 1);
                }
            }
        }
        var maxDepth = new int[n];
        foreach (var p in perm.AsEnumerable().Reverse()) {
            var d = depth[p];

            foreach (var next in adj[p]) {
                if (parent[p] == next) continue;
                d = Math.Max(d, maxDepth[next]);
            }
            maxDepth[p] = d;
        }
        int ans = 0;
        var takahashiPos = u;
        var aokiDepth = 0;
        var maxDistance = 0;
        while (true) {
            var currentDepth = depth[takahashiPos];
            if (aokiDepth >= currentDepth) break;
            var maxdepth = maxDepth[takahashiPos];

            maxDistance = Math.Max(maxDistance, maxdepth);

            takahashiPos = parent[takahashiPos];
            aokiDepth++;
        }
        cout.WriteLine(simulate(0, depth[u], maxDistance));
    }

    private int simulate(int aoki, int takahashi, int maxDistance) {
        int ans = 0;

        while (true) {
            if (takahashi < maxDistance) {
                takahashi++;
            } else {
                takahashi--;
            }
            if (aoki == takahashi) break;
            aoki++;
            ans++;
            if (aoki == takahashi) break;
        }

        return ans;
    }
}

// PREWRITEN CODE BEGINS FROM HERE
internal partial class Solver : Scanner {
    public static void Main(string[] args) {
#if LOCAL
        byte[] inputBuffer = new byte[1000000];
        var inputStream = Console.OpenStandardInput(inputBuffer.Length);
        using (var reader = new StreamReader(inputStream, Console.InputEncoding, false, inputBuffer.Length)) {
            Console.SetIn(reader);
            new Solver(Console.In, Console.Out).Run();
        }
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver(Console.In, Console.Out).Run();
        Console.Out.Flush();
#endif
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader) {
        cin = reader;
        cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer) {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

internal static class LinqPadExtension {
    public static T Dump<T>(this T obj) {
#if LOCAL
        return LINQPad.Extensions.Dump(obj);
#else
        return obj;
#endif
    }
}

public class Scanner {
    private readonly TextReader Reader;
    private readonly Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In) {
    }

    public Scanner(TextReader reader) {
        Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size) {
        string[] array = new string[size];
        for (int i = 0; i < size; i++) {
            array[i] = Next();
        }

        return array;
    }
    public int[] NextIntArray(int size) {
        int[] array = new int[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextInt();
        }

        return array;
    }

    public long[] NextLongArray(int size) {
        long[] array = new long[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextLong();
        }

        return array;
    }

    public double[] NextDoubleArray(int size) {
        double[] array = new double[size];
        for (int i = 0; i < size; i++) {
            array[i] = NextDouble();
        }

        return array;
    }

    public string Next() {
        if (TokenQueue.Count == 0) {
            if (!StockTokens()) {
                throw new InvalidOperationException();
            }
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext() {
        if (TokenQueue.Count > 0) {
            return true;
        }

        return StockTokens();
    }

    private static readonly char[] _separator = new[] { ' ', '\t' };
    private bool StockTokens() {
        while (true) {
            string line = Reader.ReadLine();
            if (line == null) {
                return false;
            }

            string[] tokens = line.Split(_separator, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) {
                continue;
            }

            foreach (string token in tokens) {
                TokenQueue.Enqueue(token);
            }

            return true;
        }
    }
}


Measured memory_bytes: 37260