H,W = map(int,input().split())
maze = []
pad = ' ' * (W+2)
maze.append(pad)
for i in range(H):
  m = input()
  maze.append(' ' + m + ' ')
maze.append(pad)
  
def bfs(maze, x,y, score):
    queue = [(x,y,0)]
    visited = {}
    while queue:
      x,y,s = queue.pop(0)
      if maze[x][y] != '.':
        continue
      visited[(x,y)] = s
      if (x+1,y) not in visited:
        queue.append((x+1,y,s+1))
      if (x-1,y) not in visited:
        queue.append((x-1,y,s+1))
      if (x,y+1) not in visited:
        queue.append((x,y+1,s+1))
      if (x,y-1) not in visited:
        queue.append((x,y-1,s+1))
    return max(visited.values())
  
ans = -1
for h in range(1,H+1):
  for w in range(1,W+1):
    now = maze[h][w]
    if now == '#':
      continue
    count = 0
    if maze[h-1][w] == '.':
      count+=1
    if maze[h+1][w] == '.':
      count+=1
    if maze[h][w-1] == '.':
      count+=1
    if maze[h][w+1] == '.':
      count+=1
    if count > 2:
      continue
    a = bfs(maze,h,w,0)
    if a > ans:
      ans = a
print(ans)