beecrowd 1621

import queue
import sys

vetx = [-1, 0, 1, 0]
vety = [ 0, 1, 0,-1]

def bfs(x , y, n, m, mt, dist):
  dist[x][y] = 0
  q = queue.Queue()
  q.put([x, y])
  md = -1 #armazena a maior distancia da origem
  sv = [-1, -1] #armazena o vertice com maior distancia da origem
  while not q.empty():
    u = q.get()
    for i in range (4):
      nx = u[0]+vetx[i]
      ny = u[1]+vety[i]
      if(nx>=0 and nx<n and ny>=0 and ny<m and dist[nx][ny]==-1 and mt[nx][ny]=='.'):
        dist[nx][ny] = dist[u[0]][u[1]] + 1
        if(dist[nx][ny]>md):
          md = dist[nx][ny]
          sv = [nx, ny]
        q.put([nx, ny])
  return [sv, md]

if(__name__ == "__main__"):
  n, m = list(map(int,sys.stdin.readline().split(' ')))
  while n or m:
    mt = [" "] * n #crianda a matrix de char
    dist  = [[-1 for i in range(m)] for i in range(n)] # criando a matrix de distancia
    dist2 = [[-1 for i in range(m)] for i in range(n)] # criando a matrix de dintancia auxiliar
    for i in range (n): #lendo o mapa
      mt[i] = sys.stdin.readline()

    ini = [-1, -1]
    for i in range(n):
      for j in range(m):
        if(mt[i][j]=='.'):
          ini = [i, j]
          break
      if(ini[0] != -1):
        break
    ax  = bfs(ini[0], ini[1], n, m, mt, dist)
    ans = bfs(ax[0][0], ax[0][1], n, m, mt, dist2)
    sys.stdout.write(str(ans[1])+"\n")
    n, m = list(map(int,sys.stdin.readline().split(' ')))