#region いつもの
#pragma warning disable
using System;
using System.Collections.Generic;
using System.Linq;
using IEnumerable = System.Collections.IEnumerable;
using IEnumerator = System.Collections.IEnumerator;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;
using TextReader = System.IO.TextReader;
using static AtCoderProject.Global;

namespace AtCoderProject
{
    static class Global
    {
        private class ComparerReverseImpl<T> : Comparer<T> where T : IComparable<T> { public override int Compare(T y, T x) => x.CompareTo(y); public override bool Equals(object obj) => obj != null && GetType() == obj.GetType(); public override int GetHashCode() => GetType().GetHashCode(); }
        public static IComparer<T> ComparerReverse<T>() where T : IComparable<T> => new ComparerReverseImpl<T>();
        public static BigInteger ParseBigInteger(string s)
        {
            // MonoのBigInteger.Parseが遅いので自前実装
            var res = BigInteger.Zero;
            var splited = new string[(s.Length + 7) / 8];
            for (int i = 0; i < splited.Length - 1; i++)
            {
                splited[i] = s.Substring(8 * i, 8);
            }
            splited[splited.Length - 1] = s.Substring(8 * (splited.Length - 1));
            foreach (var sp in splited)
            {
                res *= (int)Math.Pow(10, sp.Length);
                res += int.Parse(sp);
            }
            return res;
        }
        public static string AllLines<T>(IEnumerable<T> source) => string.Join("\n", source);
        public static string AllJoin<T>(IEnumerable<T> source) => string.Join(" ", source);
    }
    static class Ext
    {
        public static T[][] Chunk<T>(this T[,] source)
        {
            var len0 = source.GetLength(0);
            var len1 = source.GetLength(1);
            var res = new T[len0][];
            for (int i = 0; i < len0; i++)
            {
                res[i] = new T[len1];
                for (int j = 0; j < len1; j++)
                {
                    res[i][j] = source[i, j];
                }
            }
            return res;
        }

        public static Tuple<TSource, TMax> MaxBy<TSource, TMax>
            (this IEnumerable<TSource> source, Func<TSource, TMax> maxBySelector)
            where TMax : IComparable<TMax>
        {
            TMax max;
            TSource maxByItem;

            var e = source.GetEnumerator();
            e.MoveNext();
            maxByItem = e.Current;
            max = maxBySelector(maxByItem);
            while (e.MoveNext())
            {
                var item = e.Current;
                var next = maxBySelector(item);
                if (max.CompareTo(next) < 0)
                {
                    max = next;
                    maxByItem = item;
                }
            }
            return Tuple.Create(maxByItem, max);
        }
        public static Tuple<TSource, TMin> MinBy<TSource, TMin>
            (this IEnumerable<TSource> source, Func<TSource, TMin> minBySelector)
            where TMin : IComparable<TMin>
        {
            TMin min;
            TSource minByItem;

            var e = source.GetEnumerator();
            e.MoveNext();
            minByItem = e.Current;
            min = minBySelector(minByItem);
            while (e.MoveNext())
            {
                var item = e.Current;
                var next = minBySelector(item);
                if (min.CompareTo(next) > 0)
                {
                    min = next;
                    minByItem = item;
                }
            }
            return Tuple.Create(minByItem, min);
        }
        public static IComparer<T> Reverse<T>(this IComparer<T> comparer) => Comparer<T>.Create((x, y) => comparer.Compare(y, x));
        public static Dictionary<TKey, int> GroupCount<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) => source.GroupBy(keySelector).ToDictionary(g => g.Key, g => g.Count());
        public static Dictionary<TKey, int> GroupCount<TKey>(this IEnumerable<TKey> source) => source.GroupCount(i => i);
    }
    public class ConsoleReader { private string[] ReadLineSplit() => textReader.ReadLine().Split(Array.Empty<char>(), StringSplitOptions.RemoveEmptyEntries); private string[] line = Array.Empty<string>(); private int linePosition; private TextReader textReader; public ConsoleReader(TextReader tr) { textReader = tr; } public int Int => int.Parse(String); public int Int0 => Int - 1; public long Long => long.Parse(String); public double Double => double.Parse(String); public string String { get { if (linePosition >= line.Length) { linePosition = 0; line = ReadLineSplit(); } return line[linePosition++]; } } public class SplitLine { private string[] splited; public SplitLine(ConsoleReader cr) { splited = cr.ReadLineSplit(); cr.line = Array.Empty<string>(); } public int[] Int => String.Select(x => int.Parse(x)).ToArray(); public int[] Int0 => String.Select(x => int.Parse(x) - 1).ToArray(); public long[] Long => String.Select(x => long.Parse(x)).ToArray(); public double[] Double => String.Select(x => double.Parse(x)).ToArray(); public string[] String => splited; } public SplitLine Split => new SplitLine(this); public class RepeatReader : IEnumerable<ConsoleReader> { ConsoleReader cr; int count; public RepeatReader(ConsoleReader cr, int count) { this.cr = cr; this.count = count; } public IEnumerator<ConsoleReader> GetEnumerator() => Enumerable.Repeat(cr, count).GetEnumerator(); System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator(); public string[] String => this.Select(cr => cr.String).ToArray(); public int[] Int => this.Select(cr => cr.Int).ToArray(); public int[] Int0 => this.Select(cr => cr.Int - 1).ToArray(); public long[] Long => this.Select(cr => cr.Long).ToArray(); public double[] Double => this.Select(cr => cr.Double).ToArray(); } public RepeatReader Repeat(int count) => new RepeatReader(this, count); }
    public class Program
    {
        [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)] private ConsoleReader cr;
        public Program(ConsoleReader consoleReader) { this.cr = consoleReader; }
        static void Main() => Console.WriteLine(new Program(new ConsoleReader(Console.In)).Calc());

        #endregion
        public object Calc()
        {
            var H = cr.Int;
            var W = cr.Int;
            var s = cr.Repeat(H).Select(cr => cr.String.Select(c => c == '.').ToArray()).ToArray();
            var gb = new GraphBuilder(H * W, false);
            var walls = new List<int>();

            for (int h = 0; h < H; h++)
                for (int w = 0; w < W; w++)
                {
                    if (s[h][w])
                    {
                        if (w + 1 < W && s[h][w + 1])
                            gb.Add(h * W + w, h * W + w + 1);

                        if (h + 1 < H && s[h + 1][w])
                            gb.Add(h * W + w, (h + 1) * W + w);
                    }
                    else walls.Add(h * W + w);
                }

            var res = WarshallFloyd(gb.ToArray());

            return res.Cast<long>().Where(l => l < int.MaxValue).Max();
        }
        public static long[,] WarshallFloyd(Node[] graph)
        {
            var res = new long[graph.Length, graph.Length];
            for (var i = 0; i < graph.Length; i++)
            {
                for (var j = 0; j < graph.Length; j++)
                {
                    res[i, j] = long.MaxValue / 2;
                }
                res[i, i] = 0;
                foreach (var next in graph[i].children)
                    res[i, next] = 1;
            }
            for (var k = 0; k < graph.Length; k++)
                for (var i = 0; i < graph.Length; i++)
                    for (var j = 0; j < graph.Length; j++)
                        if (res[i, j] > res[i, k] + res[k, j])
                            res[i, j] = res[i, k] + res[k, j];
            return res;
        }
    }

    class GraphBuilder
    {
        private List<int>[] roots;
        private List<int>[] children;
        public GraphBuilder(int count, bool isOriented = true)
        {
            this.roots = new List<int>[count];
            this.children = new List<int>[count];
            for (var i = 0; i < count; i++)
            {
                if (isOriented)
                {
                    this.roots[i] = new List<int>();
                    this.children[i] = new List<int>();
                }
                else
                {
                    this.roots[i] = this.children[i] = new List<int>();
                }
            }
        }
        public GraphBuilder(int count, ConsoleReader cr, int edgeCount, bool isOriented = true) : this(count, isOriented)
        {
            for (var i = 0; i < edgeCount; i++)
                this.Add(cr.Int - 1, cr.Int - 1);
        }
        public void Add(int from, int to)
        {
            children[from].Add(to);
            roots[to].Add(from);
        }
        public TreeNode[] ToTree(int root)
        {
            if (this.roots[0] != this.children[0]) throw new Exception("木には無向グラフをしたほうが良い");
            var res = new TreeNode[this.children.Length];
            res[root] = new TreeNode(root, -1, this.children[root].ToArray());

            var queue = new Queue<int>();
            foreach (var child in res[root].children)
            {
                res[child] = new TreeNode(child, root, Array.Empty<int>());
                queue.Enqueue(child);
            }

            while (queue.Count > 0)
            {
                var from = queue.Dequeue();
                if (res[from].root == -1)
                    res[from].children = this.children[from].ToArray();
                else
                {
                    var children = new List<int>(this.children[from].Count);
                    foreach (var c in this.children[from])
                        if (c != res[from].root)
                            children.Add(c);

                    res[from].children = children.ToArray();
                }

                foreach (var child in res[from].children)
                {
                    res[child] = new TreeNode(child, from, Array.Empty<int>());
                    queue.Enqueue(child);
                }
            }

            return res;
        }

        public Node[] ToArray() =>
            Enumerable
            .Zip(roots, children, (r, c) => Tuple.Create(r, c))
            .Select((t, i) => new Node(i, t.Item1.ToArray(), t.Item2.ToArray()))
            .ToArray();
    }
    public class TreeNode : IEquatable<TreeNode>
    {
        public TreeNode(int i, int root, int[] children)
        {
            this.index = i;
            this.root = root;
            this.children = children;
        }
        public int index;
        public int root;
        public int[] children;

        public override string ToString() => $"children: {string.Join(",", children)}";
        public override bool Equals(object obj)
        {
            if (obj is TreeNode)
                return this.Equals((TreeNode)obj);
            else
                return false;
        }
        public bool Equals(TreeNode other) => this.index == other.index;
        public override int GetHashCode() => this.index;
    }
    public class Node : IEquatable<Node>
    {
        public Node(int i, int[] roots, int[] children)
        {
            this.index = i;
            this.roots = roots;
            this.children = children;
        }
        public int index;
        public int[] roots;
        public int[] children;

        public override string ToString() => $"children: {string.Join(",", children)}";
        public override bool Equals(object obj)
        {
            if (obj is Node)
                return this.Equals((Node)obj);
            else
                return false;
        }
        public bool Equals(Node other) => this.index == other.index;
        public override int GetHashCode() => this.index;
    }

}


Measured memory_bytes: 16864