using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	static readonly int[][] di1=new int[][]{new int[]{-1,0},new int[]{1,0},new int[]{0,-1},new int[]{0,1}};
	static readonly int[][] di2=new int[][]{new int[]{-1,-1},new int[]{-1,0},new int[]{-1,1},new int[]{0,-1},new int[]{0,1},new int[]{1,-1},new int[]{1,0},new int[]{1,1}};
	static char[][] bm;
	static int[][] b;
	static void Main(){
		Sc sc=new Sc();
		var s=sc.Ia;
		int h=s[0],w=s[1];
		char c='#';
		bm=new char[h+2][];
		b=new int[h+2][];
		for(int i = 1;i<=h;i++) {
			bm[i]=(c+sc.S+c).ToCharArray();
			b[i]=new int[w+2];
		}
		bm[0]=bm[h+1]=new string(c,w+2).ToCharArray();
		b[0]=b[h+1]=new int[w+2];
		int ans=0;
		for(int i = 1;i<=s[0];i++) {
			for(int j = 1;j<=s[1];j++) {
				if(bm[i][j]!='.'){continue;}
				for(int k = 1;k<=h;k++) {
					b[k]=new int[w+2];
				}
				var qu=new Queue<Tuple<int,int,int>>();
				qu.Enqueue(Tuple.Create(i,j,1));
				b[i][j]=1;
				while(qu.Count>0){
					var e=qu.Dequeue();
					Fdi(e.Item1,e.Item2,(y,x)=>{
						if(bm[y][x]=='.'&&b[y][x]==0){
							qu.Enqueue(Tuple.Create(y,x,e.Item3+1));
							b[y][x]=e.Item3+1;
							ans=Max(ans,e.Item3+1);
						}
					});
				}
			}
		}
		Console.WriteLine("{0}",ans-1);
	}
	static void Fdi(int y,int x,Action<int,int> f){for(int i=0;i<4;i++){f(y+di1[i][0],x+di1[i][1]);}}
}
public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}

Measured memory_bytes: 17616