from collections import deque
import copy
def solve(maze, start, end):
  memo = []
  q = deque()
  q.append([start,[]])
  ans = 0
  while q:
    cur, path = q.popleft()
    if cur == end:
      ans = path
      break
    elif cur in memo:
      continue
    else:
      memo.append(cur)
      for y, x in zip((0,0,1,-1),(1,-1,0,0)):
        if maze[cur[0]+y][cur[1]+x] == ".":
          q.append([[cur[0]+y,cur[1]+x],copy.deepcopy(path)+[cur]])
  return ans


H, W = map(int, input().split())
Maze = [["#"]*(W+2)]+[["#"]+list(input())+["#"] for _ in range(H)] + [["#"]*(W+2)]
Ans = 0
memo = [[False]*(W+2) for _ in range(H+2)]
for sy in range(1, H+1):
  for sx in range(1, W+1):
    if Maze[sy][sx] == "#":
      continue
    for ey in range(sy, H+1):
      for ex in range(1, W+1):
        if Maze[ey][ex] == "#" or memo[ey][ex]:
          continue
        P = solve(Maze, [sy, sx], [ey,ex])
        Ans = max(Ans, len(P))
        for p in P:
          memo[p[0]][p[1]] = True
        memo[ey][ex] = True
    memo[sy][sx] = True
print(Ans)