using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
 
namespace AtCoder
{
    static class Program_D
    {
        class Solver
        {
            int ri => sc.Integer();
            long rl => sc.Long();
            double rd => sc.Double();
            string rs => sc.Scan();
            public StreamScanner sc = new StreamScanner(Console.OpenStandardInput());
            private int[][] cell;
            private int[] s;
            private int h;
            private int w;
            private Queue<int[]> q;
            private int maxDistance;
            private int[] maxPos;


            // 151-D Maze Master
            public void Solve()
            {
                h = ri;
                w = ri;
                var sOrg = new int[h*w];
                for (int y = 0; y < h; ++y)
                {
                    var line = rs;
                    for (int x = 0; x < w; ++x)
                    {
                        sOrg[y*w + x] = line[x] == '.' ? 0 : -1;
                    }
                }



                s = new int[h*w];
                Array.Copy(sOrg, s, s.Length);

                maxPos = FindStartPos();
                maxDistance = 0;
#if true
                for (int y = 0; y < h; ++y)
                {
                    for (int x = 0; x < w; ++x)
                    {
                        if (s[Ofs(x, y)] != 0) continue;
                        s[Ofs(x, y)] = 1;
                        q = new Queue<int[]>();
                        q.Enqueue(new int[2] { x, y });
                        SolveMaze();
                        //DumpMaze();
                        Array.Copy(sOrg, s, s.Length);
                    }
                }
#else
                do
                {
                    q = new Queue<int[]>();
                    s[Ofs(maxPos[0], maxPos[1])] = 1;
                    q.Enqueue(new int[2] { maxPos[0], maxPos[1] });
                    var currentDistance = maxDistance;
                    SolveMaze();
                    DumpMaze();
                    if (maxDistance == currentDistance) break;
                    Array.Copy(sOrg,s,s.Length);
                } while (true);
#endif
                Wl(maxDistance-1);
            }

            private void DumpMaze()
            {
                var sb = new StringBuilder();
                for (int y = 0; y < h; ++y)
                {
                    for (int x = 0; x < w; ++x)
                    {
                        var val = s[y*w + x];
                        sb.Append(val >= 0 ? s[y*w + x].ToString("D2") : s[y*w + x].ToString("D"));
                    }
                    sb.AppendLine("");
                }
                Wl(sb.ToString());
                Wl($"currentDistance : {maxDistance}");
            }

            int Ofs(int x, int y)
            {
                return y*w + x;
            }

            bool IsInRange(int x, int y)
            {
                return 0 <= x && x < w && 0 <= y && y < h;
            }

            private void SolveMaze()
            {
                do
                {
                    //先にマスを塗ってからEnqueueした方がロスが少なそう。
                    var pos = q.Dequeue();
                    var nextDistance = s[Ofs(pos[0], pos[1])]+1;

                    var dir = new[] {new[] {1, 0}, new[] {-1, 0}, new[] {0, 1}, new[] {0, -1}};
                    for (int i = 0; i < dir.Length; ++i)
                    {
                        var next = new int[2];
                        next[0] = pos[0] + dir[i][0];
                        next[1] = pos[1] + dir[i][1];
                        if (!IsInRange(next[0], next[1])) continue;
                        if (s[Ofs(next[0], next[1])] != 0) continue;
                        s[Ofs(next[0], next[1])] = nextDistance;
                        if (nextDistance > maxDistance)
                        {
                            maxDistance = nextDistance;
                            maxPos[0] = next[0];
                            maxPos[1] = next[1];
                        }
                        q.Enqueue(next);
                    }

                } while (q.Count > 0);
            }

            private int[] FindStartPos()
            {
                var start = new int[2];
                for (int y = 0; y < h; ++y)
                {
                    for (int x = 0; x < w; ++x)
                    {
                        if (s[Ofs(x,y)] == 0)
                        {
                            start[0] = x;
                            start[1] = y;
                            return start;
                        }
                    }
                }
                throw new InvalidOperationException();
            }


            // ----------------------------------------------------------------------------
            static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }


            string Rl()
            {
                return Console.ReadLine();
            }
            void Wl(UInt64 val)
            {
                Console.WriteLine(val);
            }
            void Wl(Int64 val)
            {
                Console.WriteLine(val);
            }
            void Wl(uint val)
            {
                Console.WriteLine(val);
            }
            void Wl(int val)
            {
                Console.WriteLine(val);
            }
            void Wl(double val)
            {
                Console.WriteLine(val);
            }
            void Wl(string mes)
            {
                Console.WriteLine(mes);
            }
            public int Int()
            {
                return int.Parse(Console.ReadLine());
            }

            public int[] Ints(int subtract)
            {
                return Console.ReadLine().Split(' ').Select(s => int.Parse(s) - subtract).ToArray();
            }
            public Int64[] Int64s(Int64 subtract)
            {
                return Console.ReadLine().Split(' ').Select(s => Int64.Parse(s) - subtract).ToArray();
            }
            public UInt64[] UInt64s(UInt64 subtract)
            {
                return Console.ReadLine().Split(' ').Select(s => UInt64.Parse(s) - subtract).ToArray();
            }


        }



        static void Main(string[] args)
        {
            //Console.SetOut(new Printer(Console.OpenStandardOutput()) { AutoFlush = false });
            new Solver().Solve();
            //Console.Out.Flush();
        }
    }

    public class Printer : StreamWriter
    {
        public override IFormatProvider FormatProvider => CultureInfo.InvariantCulture;
        public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
    }

    public class StreamScanner
    {
        public StreamScanner(Stream stream) { str = stream; }

        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;

        private byte read()
        {
            if (isEof) return 0;
            if (ptr >= len)
            {
                ptr = 0;
                if ((len = str.Read(buf, 0, 1024)) <= 0)
                {
                    isEof = true;
                    return 0;
                }
            }
            return buf[ptr++];
        }

        public char Char()
        {
            byte b = 0;
            do b = read(); while ((b < 33 || 126 < b) && !isEof);
            return (char)b;
        }
        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read()) sb.Append(b);
            return sb.ToString();
        }
        public string ScanLine()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n' && b != 0; b = (char)read()) if (b != '\r') sb.Append(b);
            return sb.ToString();
        }
        public long Long() { return isEof ? long.MinValue : long.Parse(Scan()); }
        public int Integer() { return isEof ? int.MinValue : int.Parse(Scan()); }
        public double Double() { return isEof ? double.NaN : double.Parse(Scan(), CultureInfo.InvariantCulture); }
    }

    struct ModInt
    {
        const int MOD = 1000000007;
        long Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public ModInt(long data) { if ((Data = data % MOD) < 0) Data += MOD; }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator long(ModInt modInt) => modInt.Data;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static implicit operator ModInt(long val) => new ModInt(val);
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, ModInt b)
        {
            long res = a.Data + b.Data;
            return new ModInt() { Data = res >= MOD ? res - MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator +(ModInt a, long b) => a.Data + b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, ModInt b)
        {
            long res = a.Data - b.Data;
            return new ModInt() { Data = res < 0 ? res + MOD : res };
        }
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator -(ModInt a, long b) => a.Data - b;
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator *(ModInt a, ModInt b) => new ModInt() { Data = a.Data * b.Data % MOD };
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ModInt operator /(ModInt a, ModInt b) => a.Data * GetInverse(b);

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public override string ToString() => Data.ToString();

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        static long GetInverse(long a)
        {
            long div;
            long p = MOD;
            long x1 = 1, y1 = 0, x2 = 0, y2 = 1;
            while (true)
            {
                if (p == 1) return x2;
                div = a / p;
                x1 -= x2 * div;
                y1 -= y2 * div;
                a %= p;
                if (a == 1) return x1;
                div = p / a;
                x2 -= x1 * div;
                y2 -= y1 * div;
                p %= a;
            }
        }
    }

}

Measured memory_bytes: 45924