from collections import deque
h,w=map(int,input().split())
meiro=[]
for i in range(h):
	tmp=input()
	meiro.append([tmp[i] for i in range(w)])



def bfs(sh,sw,eh,ew,meiro):
	que=deque([[sh,sw]])
	dist=[[-1 for _ in range(w)] for _ in range(h)]
	dist[sh][sw]=0
	
	while True:
		check=que.popleft()
		hnow=check[0]
		wnow=check[1]
		
		dnow=dist[hnow][wnow]
		
		if hnow==eh and wnow== ew:
			return dnow
		
		hnext=hnow-1
		wnext=wnow
		if 0 < hnow:
			if dist[hnext][wnext]==-1 and meiro[hnext][wnext] == ".":
				que.append([hnext,wnext])
				dist[hnext][wnext]=dnow+1
		
		hnext=hnow+1
		wnext=wnow
		if h-1 > hnow:
			if dist[hnext][wnext]==-1 and meiro[hnext][wnext] == ".":
				que.append([hnext,wnext])
				dist[hnext][wnext]=dnow+1
		
		hnext=hnow
		wnext=wnow-1
		if 0 < wnow:
			if dist[hnext][wnext]==-1 and meiro[hnext][wnext] == ".":
				que.append([hnext,wnext])
				dist[hnext][wnext]=dnow+1
		
		hnext=hnow
		wnext=wnow+1
		if w-1 > wnow:
			if dist[hnext][wnext]==-1 and meiro[hnext][wnext] == ".":
				que.append([hnext,wnext])
				dist[hnext][wnext]=dnow+1

roadlist=[]
for i in range(h):
	for j in range(w):
		if meiro[i][j] == ".":
			roadlist.append([i,j])

max=0
for start in roadlist:
	for end in roadlist:
		tmp=bfs(start[0],start[1],end[0],end[1],meiro)
		if tmp > max:
			max = tmp

print(max)


