using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Threading;
using System.Text;
using System.Diagnostics;
using static util;
using P = pair<int, int>;
using Number = System.Int64;

class Program {
    static void Main(string[] args) {
        var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
        var solver = new Solver(sw);
        // var t = new Thread(solver.solve, 1 << 26); // 64 MB
        // t.Start();
        // t.Join();
        solver.solve();
        sw.Flush();
    }
}

class Solver {
    StreamWriter sw;
    Scan sc;
    void Prt(string a) => sw.WriteLine(a);
    void Prt<T>(IEnumerable<T> a) => Prt(string.Join(" ", a));
    void Prt(params object[] a) => Prt(string.Join(" ", a));
    public Solver(StreamWriter sw) {
        this.sw = sw;
        this.sc = new Scan();
    }

    public void solve() {
        int h, w;
        sc.Multi(out h, out w);
        var s = new string[h];
        var dist = new long[h * w][];
        for (int i = 0; i < h * w; i++)
        {
            dist[i] = new long[h * w];
            for (int j = 0; j < h * w; j++)
            {
                dist[i][j] = M;
            }
            dist[i][i] = 0;
        }
        for (int i = 0; i < h; i++)
        {
            s[i] = sc.Str;
        }
        for (int i = 0; i < h; i++)
        {
            for (int j = 0; j < w; j++)
            {
                if (s[i][j] == '#') continue;
                foreach (var item in adjacents(i, j, h, w))
                {
                    if (s[item.v1][item.v2] == '.') {
                        dist[i * w + j][item.v1 * w + item.v2] = 1;
                    }
                }
            }
        }
        ShortestPath.WarshallFloyd(dist);
        long ans = 0;
        foreach (var item in dist)
        {
            foreach (var i in item)
            {
                if (i < M) {
                    ans = Math.Max(ans, i);
                }
            }
        }
        Prt(ans);

    }
}

class pair<T, U> : IComparable<pair<T, U>> {
    public T v1;
    public U v2;
    public pair() : this(default(T), default(U)) {}
    public pair(T v1, U v2) { this.v1 = v1; this.v2 = v2; }
    public int CompareTo(pair<T, U> a) {
        int c = Comparer<T>.Default.Compare(v1, a.v1);
        return c != 0 ? c : Comparer<U>.Default.Compare(v2, a.v2);
    }
    public override string ToString() => v1 + " " + v2;
    public void Deconstruct(out T a, out U b) { a = v1; b = v2; }
}
static class util {
    public static readonly int M = 1000000007;
    // public static readonly int M = 998244353;
    public static readonly long LM = 1L << 60;
    public static readonly double eps = 1e-11;
    public static void DBG(string a) => Console.Error.WriteLine(a);
    public static void DBG<T>(IEnumerable<T> a) => DBG(string.Join(" ", a));
    public static void DBG(params object[] a) => DBG(string.Join(" ", a));
    public static void Assert(bool cond) { if (!cond) throw new Exception(); }
    public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
    public static int CompareList<T>(IList<T> a, IList<T> b) where T : IComparable<T> {
        for (int i = 0; i < a.Count && i < b.Count; i++)
            if (a[i].CompareTo(b[i]) != 0) return a[i].CompareTo(b[i]);
        return a.Count.CompareTo(b.Count);
    }
    public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w;
    static readonly int[] dd = { 0, 1, 0, -1 };
    // static readonly string dstring = "RDLU";
    public static P[] adjacents(int i, int j)
        => Enumerable.Range(0, dd.Length).Select(k => new P(i + dd[k], j + dd[k ^ 1])).ToArray();
    public static P[] adjacents(int i, int j, int h, int w)
        => Enumerable.Range(0, dd.Length).Select(k => new P(i + dd[k], j + dd[k ^ 1]))
                                         .Where(p => inside(p.v1, p.v2, h, w)).ToArray();
    public static P[] adjacents(this P p) => adjacents(p.v1, p.v2);
    public static P[] adjacents(this P p, int h, int w) => adjacents(p.v1, p.v2, h, w);
    public static Dictionary<T, int> compress<T>(this IEnumerable<T> a)
        => a.Distinct().OrderBy(v => v).Select((v, i) => new { v, i }).ToDictionary(p => p.v, p => p.i);
    public static Dictionary<T, int> compress<T>(params IEnumerable<T>[] a) => compress(a.Aggregate(Enumerable.Union));
    public static T[] inv<T>(this Dictionary<T, int> dic) {
        var res = new T[dic.Count];
        foreach (var item in dic) res[item.Value] = item.Key;
        return res;
    }
    public static void swap<T>(ref T a, ref T b) where T : struct { var t = a; a = b; b = t; }
    public static void swap<T>(this IList<T> a, int i, int j) where T : struct { var t = a[i]; a[i] = a[j]; a[j] = t; }
    public static T[] copy<T>(this IList<T> a) {
        var ret = new T[a.Count];
        for (int i = 0; i < a.Count; i++) ret[i] = a[i];
        return ret;
    }
}

class Scan {
    StreamReader sr;
    public Scan() { sr = new StreamReader(Console.OpenStandardInput()); }
    public Scan(string path) { sr = new StreamReader(path); }
    public int Int => int.Parse(Str);
    public long Long => long.Parse(Str);
    public double Double => double.Parse(Str);
    public string Str => sr.ReadLine().Trim();
    public pair<T, U> Pair<T, U>() {
        T a; U b;
        Multi(out a, out b);
        return new pair<T, U>(a, b);
    }
    public P P => Pair<int, int>();
    public int[] IntArr => StrArr.Select(int.Parse).ToArray();
    public long[] LongArr => StrArr.Select(long.Parse).ToArray();
    public double[] DoubleArr => StrArr.Select(double.Parse).ToArray();
    public string[] StrArr => Str.Split(new[]{' '}, StringSplitOptions.RemoveEmptyEntries);
    bool eq<T, U>() => typeof(T).Equals(typeof(U));
    T ct<T, U>(U a) => (T)Convert.ChangeType(a, typeof(T));
    T cv<T>(string s) => eq<T, int>()    ? ct<T, int>(int.Parse(s))
                       : eq<T, long>()   ? ct<T, long>(long.Parse(s))
                       : eq<T, double>() ? ct<T, double>(double.Parse(s))
                       : eq<T, char>()   ? ct<T, char>(s[0])
                                         : ct<T, string>(s);
    public void Multi<T>(out T a) => a = cv<T>(Str);
    public void Multi<T, U>(out T a, out U b)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); }
    public void Multi<T, U, V>(out T a, out U b, out V c)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); }
    public void Multi<T, U, V, W>(out T a, out U b, out V c, out W d)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]); }
    public void Multi<T, U, V, W, X>(out T a, out U b, out V c, out W d, out X e)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]); e = cv<X>(ar[4]); }
}


static class ShortestPath {
    public static void WarshallFloyd(Number[][] g) {
        int n = g.Length;
        for (int i = 0; i < n; ++i)
            for (int j = 0; j < n; ++j)
                for (int k = 0; k < n; ++k)
                    g[j][k] = Math.Min(g[j][k], g[j][i] + g[i][k]);
    }

    class Heap {
        int n;
        Number[] values;
        int[] keys, indices;

        void Update(int i, Number val) {
            if (indices[i] == -1) {
                keys[n] = i;
                indices[i] = n;
                ++n;
            }
            values[i] = val;
            int p = indices[i];
            while (p > 0) {
                int par = p - 1 >> 1;
                if (values[keys[par]] > val) {
                    keys[p] = keys[par];
                    indices[keys[p]] = p;
                    p = par;
                }
                else break;
            }
            keys[p] = i;
            indices[i] = p;
        }
        int Pop() {
            --n;
            int ret = keys[0];
            indices[ret] = -1;
            int i = keys[n];
            keys[n] = -1;
            Number val = values[i];
            if (n == 0) return ret;
            int p = 0;
            while ((p << 1 | 1) < n) {
                int ch = p << 1 | 1;
                if (ch < n - 1 && values[keys[ch]] > values[keys[ch + 1]]) ++ch;
                if (val > values[keys[ch]]) {
                    keys[p] = keys[ch];
                    indices[keys[p]] = p;
                    p = ch;
                }
                else break;
            }
            keys[p] = i;
            indices[i] = p;
            return ret;
        }

        void init(int m, int s, Number inf) {
            values = new Number[m];
            keys = new int[m];
            indices = new int[m];
            for (int i = 0; i < m; i++) {
                values[i] = inf;
                keys[i] = -1;
                indices[i] = -1;
            }
            values[s] = 0;
            keys[0] = s;
            indices[s] = 0;
        }

        public Number[] Run(List<pair<Number, int>>[] edges, int s, Number inf) {
            init(edges.Length, s, inf);

            n = 1;
            while (n > 0) {
                int p = Pop();
                foreach (var e in edges[p])
                    if (values[e.v2] > values[p] + e.v1)
                        Update(e.v2, values[p] + e.v1);
            }
            return values;
        }
        public pair<Number[], long[]> Run(List<pair<Number, int>>[] edges, int s, long Mod, Number inf) {
            init(edges.Length, s, inf);
            var cnts = new long[edges.Length];
            cnts[s] = 1;

            n = 1;
            while (n > 0) {
                int p = Pop();
                foreach (var e in edges[p])
                    if (values[e.v2] > values[p] + e.v1) {
                        cnts[e.v2] = cnts[p];
                        Update(e.v2, values[p] + e.v1);
                    }
                    else if (values[e.v2] == values[p] + e.v1)
                        cnts[e.v2] = (cnts[e.v2] + cnts[p]) % Mod;
            }
            return new pair<Number[], long[]>(values, cnts);
        }
    }

    public static Number[] Dijkstra(List<pair<Number, int>>[] edges, int s, Number inf)
        => new Heap().Run(edges, s, inf);

    public static pair<Number[], long[]> Dijkstra(List<pair<Number, int>>[] edges, int s, long Mod, Number inf)
        => new Heap().Run(edges, s, Mod, inf);
}


Measured memory_bytes: 13652