using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using static System.Math;

namespace CSharpSpiral {
	class Program {
		static void Main(string[] args) {
			var sw = new StreamWriter(Console.OpenStandardOutput()) {
				AutoFlush=
#if DEBUG
			true
#else
			false
#endif
			};
			Console.SetOut(sw);
			new Program().Solve(new ConsoleInput(
#if DEBUG
			new StreamReader("../../../inputData.txt")
#else
			Console.In
#endif
			, ' '));
			Console.Out.Flush();
		}
		public void Solve(ConsoleInput cin) {
			var H = cin.ReadInt;
			var W = cin.ReadInt;
			var S = new bool[H, W];
			var G = new Graph(H*W);
			var firstH = 0;
			var firstW = 0;
			var landFound = false;
			for(int i = 0; i<H; i++) {
				var line = cin.ReadLine;
				for(int j = 0; j<W; j++) {
					S[i, j]=line[j]=='.';
					if(!landFound&&S[i, j]) {
						firstH=i; firstW=j;
						landFound=true;
					}
				}
			}
			for(int i = 0; i<H; i++) {
				for(int j = 0; j<W-1; j++) {
					if(S[i, j]&&S[i, j+1]) {
						G.AddEdge(i*W+j, i*W+j+1, 1);
						G.AddEdge(i*W+j+1, i*W+j, 1);
					}
				}
			}
			for(int i = 0; i<H-1; i++) {
				for(int j = 0; j<W; j++) {
					if(S[i, j]&&S[i+1, j]) {
						G.AddEdge(i*W+j, (i+1)*W+j, 1);
						G.AddEdge((i+1)*W+j, i*W+j, 1);
					}
				}
			}
			int[] tree;
			var IJK = G.Dijkstra(firstH*W+firstW, out tree);
			var M = IJK.Max();
			var farest = IJK.ToList().IndexOf(M);
			var Res = G.Dijkstra(farest, out tree).Max();
			Console.WriteLine(Res);
		}
	}
	static class ExtendClass {
		public static string Join<T>(this IEnumerable<T> X, string separator = " ") => string.Join(separator, X);
		public static void MergeSort<T>(this T[] A, bool desending) where T : IComparable<T> {
			if(desending)
				MergeSort(A, Comparer<T>.Create((x, y) => y.CompareTo(x)));
			else
				MergeSort(A, Comparer<T>.Create((x, y) => x.CompareTo(y)));
		}
		public static void MergeSort<T>(T[] A, IComparer<T> comparer) where T : IComparable<T> {
			MergeSort1(0, A.Length, A, comparer);
		}
		private static void Merge<T>(int left, int mid, int right, T[] A, IComparer<T> comparer)
			where T : IComparable<T> {
			var nl = mid-left;
			var nr = right-mid;
			var L = new T[nl];
			var R = new T[nr];
			for(int i = 0; i<nl; i++)
				L[i]=A[i+left];
			for(int i = 0; i<nr; i++)
				R[i]=A[i+mid];
			int il = 0, ir = 0;
			for(int k = left; k<right; k++) {
				if(comparer.Compare(L[il], R[ir])<=0) {
					A[k]=L[il++];
					if(il==nl) {
						k++;
						for(; ir<nr;)
							A[k++]=R[ir++];
						break;
					}
				} else {
					A[k]=R[ir++];
					if(ir==nr) {
						k++;
						for(; il<nl;)
							A[k++]=L[il++];
						break;
					}
				}
			}
		}
		private static void MergeSort1<T>(int left, int right, T[] A, IComparer<T> comparer)
		where T : IComparable<T> {
			if(left+1<right) {
				var mid = (left+right)/2;
				MergeSort1(left, mid, A, comparer);
				MergeSort1(mid, right, A, comparer);
				Merge(left, mid, right, A, comparer);
			}
		}
	}
	class ConsoleInput {
		private readonly TextReader _stream;
		private readonly char _separator = ' ';
		private readonly Queue<string> inputStream;
		public ConsoleInput(TextReader stream, char separator = ' ') {
			this._separator=separator;
			this._stream=stream;
			inputStream=new Queue<string>();
		}
		public string Read {
			get {
				if(inputStream.Count!=0) return inputStream.Dequeue();
				string[] tmp = _stream.ReadLine().Split(_separator);
				for(int i = 0; i<tmp.Length; ++i)
					inputStream.Enqueue(tmp[i]);
				return inputStream.Dequeue();
			}
		}
		public string ReadLine { get { return _stream.ReadLine(); } }
		public int ReadInt { get { return int.Parse(Read); } }
		public long ReadLong { get { return long.Parse(Read); } }
		public double ReadDouble { get { return double.Parse(Read); } }
		public string[] ReadStrArray(long N) {
			var ret = new string[N];
			for(long i = 0; i<N; ++i)
				ret[i]=Read;
			return ret;
		}
		public int[] ReadIntArray(long N) { var ret = new int[N]; for(long i = 0; i<N; ++i) ret[i]=ReadInt; return ret; }
		public long[] ReadLongArray(long N) { var ret = new long[N]; for(long i = 0; i<N; ++i) ret[i]=ReadLong; return ret; }
		public double[] ReadDoubleArray(long N) { var ret = new double[N]; for(long i = 0; i<N; ++i) ret[i]=ReadDouble; return ret; }
		public bool isEOF {
			get { return _stream.Peek()==-1; }
		}
	}
	class PriorityQueue<T> where T : IComparable<T> {
		protected readonly List<T> body = new List<T>();
		protected Comparer<T> comparer { get; }
		public PriorityQueue(bool maxTop = true) {
			comparer
			=maxTop ? Comparer<T>.Create((T i, T j) => i.CompareTo(j))
			: Comparer<T>.Create((T i, T j) => j.CompareTo(i));
		}
		public PriorityQueue(Comparer<T> mComparer) { comparer=mComparer; }
		//suppose Me>Her if isRootMaximum
		protected int Compare(T Me, T Her) => comparer.Compare(Me, Her);
		protected int Compare(int x, int y) => Compare(body[x], body[y]);
		protected virtual void Swap(int x, int y) { T tmp = body[x]; body[x]=body[y]; body[y]=tmp; }
		public virtual void Push(T elem) {
			body.Add(elem);
			//the parent of n-th element is the ((n+1)>>1)-1-th
			UpHeap(lastIndex);
		}
		public void Push(IEnumerable<T> seq) {
			foreach(var item in seq) {
				Push(item);
			}
		}
		protected int UpHeap(int j) {
			while(j>0) {
				int parent = ((j+1)>>1)-1;
				if(Compare(parent, j)<0) {
					Swap(parent, j);
				} else break;
				j=parent;
			}
			return j;
		}
		public virtual T Pop() {
			T top = body[0];
			Swap(0, lastIndex);
			body.RemoveAt(lastIndex);
			DownHeap(0);
			return top;
		}
		protected int DownHeap(int j) {
			while(j<length) {
				var leftChild = ((j+1)<<1)-1;
				if(leftChild>=length) break;
				var toSwap = leftChild;
				var rightChild = leftChild+1;
				if(rightChild<length&&Compare(leftChild, rightChild)<0)
					toSwap=rightChild;
				if(Compare(j, toSwap)<0) {
					Swap(toSwap, j);
				} else break;
				j=toSwap;
			}
			return j;
		}
		public T Top { get { return body[0]; } }
		public int length { get { return body.Count(); } }
		protected int lastIndex { get { return body.Count()-1; } }
		public bool Any() => body.Any();
		public string Emit() { return body.ToArray().Join(","); }
	}
	class ImmutablePriorityQueue<T>: PriorityQueue<T> where T : IComparable<T> {
		public Dictionary<T, int> map { get; private set; } = new Dictionary<T, int>();
		public ImmutablePriorityQueue(bool maxTop) : base(maxTop) { }
		public ImmutablePriorityQueue(Comparer<T> mComparer) : base(mComparer) { }
		protected override void Swap(int x, int y) {
			map[body[x]]=y;
			map[body[y]]=x;
			base.Swap(x, y);
		}
		public override void Push(T elem) {
			map.Add(elem, length);
			base.Push(elem);
		}
		public override T Pop() {
			var V = base.Pop();
			map.Remove(V);
			return V;
		}
		public void Remove(T X) {
			var x = map[X];
			map.Remove(X);
			Swap(x, lastIndex);
			body.RemoveAt(lastIndex);
			var b = DownHeap(x);
			if(b==x) UpHeap(x);
		}
		public void Update(T X) {
			var x = map[X];
			var b = DownHeap(x);
			if(b==x) UpHeap(x);
		}
		public bool hasKey(T X) { int c; return map.TryGetValue(X, out c); }
	}
	class Graph: Graph<long> {
		public Graph(int _N) : base(_N) { }
		public long[] Dijkstra(int start, out int[] froms) {
			var tmp = new long?[N];
			froms=new int[N];
			var PQComparer = Comparer<int>.Create(
				(i, j) => {
					if(tmp[i].HasValue) {
						return tmp[j].HasValue ? -Nullable.Compare(tmp[i], tmp[j]) : 1;
					} else {
						return tmp[j].HasValue ? -1 : 0;
					}
				}
				);
			//top is Min, null is ""larger"" than others
			var PQ = new ImmutablePriorityQueue<int>(PQComparer);//manages ids of nodes
			var Picked = new bool[N];
			tmp[start]=0;
			for(int i = 0; i<N; i++)
				froms[i]=-1;
			froms[start]=-1;
			for(int i = 0; i<N; i++)
				PQ.Push(i);
			while(PQ.Any()) {
				var v = PQ.Pop();
				Picked[v]=true;
				//TODO
				foreach(var wc in Edges[v]) {
					var w = wc.Key;
					if(Picked[w]) continue;
					var c = wc.Value;
					if(tmp[w]==null||tmp[w]>tmp[v]+c) {
						tmp[w]=tmp[v]+c;
						PQ.Update(w);
						froms[w]=v;
					}
				}
			}
			return tmp.Select(x => x??-1).ToArray();
		}
	}
	class Graph<T> {
		public readonly int N;
		public T[] Nodes { get; }
		protected List<int>[] Nexts { get; }
		protected Dictionary<int, T>[] Edges { get; set; }
		public Graph(int _N) {
			N=_N;
			Nodes=new T[_N];
			Nexts=new List<int>[_N];
			Edges=new Dictionary<int, T>[_N];
			for(int i = 0; i<_N; i++) {
				Nexts[i]=new List<int>();
				Edges[i]=new Dictionary<int, T>();
			}
		}
		public void Update(int i, T value) {
			Nodes[i]=value;
		}
		public void AddEdge(int from, int to, T value) {
			Edges[from].Add(to, value);
			Nexts[from].Add(to);
		}
		private void PointDFS(int from, Action<int> onEnter, Action<int> onLeave) {
			var S = new Stack<int>();
			S.Push(from);
			var entered = new bool[N];
			var departed = new bool[N];//dep<=ent
			var childCount = new int[N];
			while(S.Any()) {
				var v = S.Peek();
				if(!entered[v]) {
					onEnter?.Invoke(v);
					entered[v]=true;
				}
				for(int i = childCount[v]; i<Nexts[v].Count; i++) {
					if(!entered[Nexts[v][i]]) {
						S.Push(Nexts[v][i]);
						childCount[v]=i+1;
						goto NextLoop;
					}
				}
				S.Pop();
				childCount[v]=Nexts[v].Count;
				if(!departed[v]) {
					onLeave?.Invoke(v);
					departed[v]=true;
				}
			NextLoop:;
			}
		}
	}
}