using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Numerics;

namespace AtCoderWorkspace
{
    public class Solver
    {
        public void Solve()
        {
            var cin = new Scanner();
            
            var H = cin.nextInt();
            var W = cin.nextInt();

            var grid = new List<List<char>>();
            grid.Add(Enumerable.Repeat<char>('#', W + 2).ToList());
            for (int i = 0; i < H; i++)
            {
                var row = "#" + cin.next() + "#";
                grid.Add(row.ToCharArray().ToList());
            }
            grid.Add(Enumerable.Repeat<char>('#', W + 2).ToList());
                       
            var ans = 0;
            var vx = new int[] { 0, 1, 0, -1 };
            var vy = new int[] { -1, 0, 1, 0 };
            
            for (int y =1; y <= H; y++)
            {
                for (int x = 1; x <= W; x++)
                {
                    if (grid[y][x] == '#') continue;
                    var que = new Queue<Point>();
                    que.Enqueue(new Point() { X = x, Y = y });
                    var visited = new bool[H, W];
                    visited[y - 1, x - 1] = true;
                    while (que.Count > 0)
                    {
                        var p = que.Dequeue();
                        ans = Math.Max(ans, p.Depth);
                        

                        for (int i = 0; i < 4; i++)
                        {
                            var nextX = p.X + vx[i];
                            var nextY = p.Y + vy[i];
                            var nextDepth = p.Depth+1;

                            if (grid[nextY][nextX] == '#') continue;
                            if (visited[nextY-1, nextX-1] == true) continue;

                            que.Enqueue(new Point()
                            {
                                X = nextX,
                                Y = nextY,
                                Depth = nextDepth
                            });
                            visited[nextY - 1, nextX - 1] = true;
                        }                        
                    }
                }
            }

            Console.WriteLine(ans);
        }
    }

    class Point
    {
        public int X { get; set; }
        public int Y { get; set; }
        public int Depth { get; set; }
    }

    class Scanner
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int nextInt()
        {
            return int.Parse(next());
        }
        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt() + add;
            }
            return Array;
        }

        public int[] DPArrayInt(int N, int ex)
        {
            int[] Array = new int[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt();
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong() + add;
            }
            return Array;
        }

        public long[] DPArrayLong(int N, int ex)
        {
            long[] Array = new long[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong();
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }


        public double[] ArrayDouble(int N, double add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextDouble() + add;
            }
            return Array;
        }
    }

    class Program
    {
        static void Main(string[] s) => new Solver().Solve();
    }
}


Measured memory_bytes: 15568