h,w = map(int,input().split())
maze = [input() for _ in range(h)]
from collections import deque
D = [(0,1),(1,0),(-1,0),(0,-1)]
ans = 0
for i in range(h):
  for j in range(w):
    queue  = deque([(i,j,0)])
    checked = [[0 for _ in range(w)] for _ in range(h)]
    checked[i][j] = 1
    while queue:
      j,k,d = queue.popleft()
      for dx,dy in D:
        x,y = j+dx,k+dy
        if not(0<=x<h) or not(0<=y<w):
          continue
        if maze[x][y] == '#':
          continue
        if checked[x][y]:
          continue
        checked[x][y] = d+1
        queue.append((x,y,d+1))
    for j in range(h):
      for k in range(w):
        if checked:
          ans = max(ans,checked[j][k])

print(ans)