using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using static kyoupuro.Input;
using static kyoupuro.MyMath;

namespace kyoupuro {

    class MainClass {

        public static void Main() {
            int H = NextInt, W = NextInt;
            var data = new List<string>();
            var d = new int[H * W, H * W];

            for (int i = 0; i < H * W; i++) {
                for (int j = 0; j < H * W; j++) {
                    d[i, j] = 100000000;
                }
            }

            for (int i = 0; i < H; i++) {
                data.Add(Line);
                for (int j = 0; j < W; j++) {
                    if (data[i][j] == '.') d[i * W + j, i * W + j] = 0;
                }
            }

            for (int i = 0; i < H; i++) {
                for (int j = 0; j < W; j++) {
                    int pos = i * W + j;

                    if (data[i][j] == '#') continue;

                    if (j + 1 < W && data[i][j + 1] == '.') d[pos, pos + 1] = 1;
                    if (i + 1 < H && data[i + 1][j] == '.') d[pos, pos + W] = 1;
                    if (i > 0 && data[i - 1][j] == '.') d[pos, pos - W] = 1;
                    if (j > 0 && data[i][j - 1] == '.') d[pos, pos - 1] = 1;
                }
            }

            for (int k = 0; k < H * W; k++) {
                for (int i = 0; i < H * W; i++) {
                    for (int j = 0; j < H * W; j++) {
                        d[i, j] = Math.Min(d[i, j], d[i, k] + d[k, j]);
                    }
                }
            }

            var max = 0;
            for (int i = 0; i < H * W; i++) {
                for (int j = 0; j < H * W; j++) {
                    if (d[i, j] < 10000 && max < d[i, j]) {
                        max = d[i, j];
                    }
                }
            }

            /*
            for (int i = 0; i < H * W; i++) {
                for (int j = 0; j < W * H; j++) {
                    Console.Write(d[i, j] + " ");
                }
                Console.WriteLine();
            }*/

            Console.WriteLine(max);
        }
    }

    class MyMath {
        public const long mod = 1000000007;

        public static long Gcd(long a, long b) {
            return b == 0 ? a : Gcd(b, a % b);
        }

        public static long Sqrt(long n) {
            if (n < 0) return 0;
            long a = 0, tmp = 0, b = 0;
            for (int i = 62; i >= 0; i -= 2) {
                tmp = (b << 1) + 1 <= (n >> i) ? 1 : 0;
                a = a << 1 | tmp;
                n -= (((b << 1) + 1) * tmp) << i;
                b = (b << 1) + tmp + tmp;
            }
            return a;
        }

        public static long Factorial(int n) {
            if (n == 0)
                return 1L;
            return n * Factorial(n - 1);
        }
    }

    public class PriorityQueue<T> {

        readonly List<T> _heap = new List<T>();
        readonly Comparison<T> _comparison;

        public PriorityQueue(Comparison<T> comparison) {
            _comparison = comparison;
        }

        public PriorityQueue() : this(Comparer<T>.Default.Compare) { }

        public PriorityQueue(IComparer<T> comparer) : this(comparer.Compare) { }

        public void Enqueue(T item) {
            _heap.Add(item);

            int childID = _heap.Count - 1;
            int parentID = (childID - 1) / 2;

            while (childID > 0 && _comparison(_heap[parentID], _heap[childID]) > 0) {
                Swap(parentID, childID);
                childID = parentID;
                parentID = (parentID - 1) / 2;
            }
        }

        public T Dequeue() {
            var first = _heap.First();
            _heap[0] = _heap.Last();
            _heap.RemoveAt(_heap.Count - 1);

            int parentID = 0;
            int childID = parentID * 2 + 2;
            if (childID >= _heap.Count || _comparison(_heap[childID], _heap[childID - 1]) > 0) childID--;

            while (childID < _heap.Count && _comparison(_heap[parentID], _heap[childID]) > 0) {
                Swap(parentID, childID);
                parentID = childID;
                childID = parentID * 2 + 2;
                if (childID >= _heap.Count || _comparison(_heap[childID], _heap[childID - 1]) > 0) childID--;
            }

            return first;
        }

        void Swap(int parent, int child) {
            var tmp = _heap[parent];
            _heap[parent] = _heap[child];
            _heap[child] = tmp;
        }

        public T Peek() { return _heap[0]; }
        public int Count => _heap.Count;
        public bool Any() { return _heap.Any(); }
        public List<T> Data => _heap;
    }

    public class UnionFind {
        public int[] Parents { get; set; }
        public UnionFind(int size) {
            Parents = Enumerable.Repeat(-1, size).ToArray();
        }

        public int Find(int num) {
            if (Parents[num] < 0) return num;

            Parents[num] = Find(Parents[num]);
            return Parents[num];
        }

        public int Size(int num) {
            return -Parents[Find(num)];
        }

        public bool Same(int a, int b) {
            return Find(a) == Find(b);
        }

        public void Unite(int a, int b) {
            int x = Find(a), y = Find(b);

            if (Size(x) > Size(y)) {
                Parents[x] += Parents[y];
                Parents[y] = x;
            } else {
                Parents[y] += Parents[x];
                Parents[x] = y;
            }
        }
    }

    class Input {
        static IEnumerator<string> enumerator = new string[] { }.AsEnumerable().GetEnumerator();

        public static string Line => Console.ReadLine();

        public static string[] StrArr => Line.Split(' ');

        public static int NextInt => int.Parse(NextWord());

        public static long NextLong => long.Parse(NextWord());

        public static List<int> IntList => StrArr.Select(int.Parse).ToList();

        public static List<long> LongList => StrArr.Select(long.Parse).ToList();

        public static string NextWord() {
            while (!enumerator.MoveNext()) {
                enumerator = StrArr.AsEnumerable().GetEnumerator();
            }
            return enumerator.Current;
        }
    }
}

Measured memory_bytes: 13272