from collections import deque
H, W = map(int, input().split())
sheet = [[0]*(W+2) for i in range(H+2)]
for i in range(H):
  s = input()
  for j in range(W):
    if s[j]=='.':
      sheet[i+1][j+1] = 1

ans = -1
for i in range(1, H+1):
  for j in range(1, W+1):
    m = -1
    if sheet[i][j]==0:
      continue
    stuck = deque([(j, i, 0)])
    log = [[0]*(W+2) for i in range(H+2)]
    while stuck:
      x, y, c = stuck.popleft()
      if c>m:
        m = c
      for dx, dy in [(0,1), (0,-1), (1,0), (-1,0)]:
        if sheet[y+dy][x+dx]==1 and log[y+dy][x+dx]==0:
          log[y+dy][x+dx] = 1
          stuck += [(x+dx, y+dy, c+1)]
    if ans<m:
      ans = m
print(ans)