from collections import deque
import math
import copy
H,W = map(int,input().split())
maze = [list(input()) for _ in range(H)]
def bfs(maze,h,w,sx,sy):
    for y in range(h):
        for x in range(w):
            if maze[y][x]=='.':
                maze[y][x]=float('inf')

    maze[sy][sx]=0
    que=deque([[sy,sx]])
    while que:
        y,x=que.popleft()
        for i,j in [(1,0),(0,1),(-1,0),(0,-1)]:
            if 0 <= y+i < H and 0<=x+j<W:
                nexty,nextx=y+i,x+j
                dist=maze[nexty][nextx]
                if dist!='#':
                    if dist>maze[y][x]+1:
                        maze[nexty][nextx]=maze[y][x]+1
                        que.append([nexty,nextx])
    m = 0                   
    for l in maze:
      m = max(m,max([i for i in l if i != '#' and i < 1000000]))
    return m
max_dist = -1  
for si in range(H):
  for sj in range(W):
    if maze[si][sj] == '#':
      continue
    copymaze = copy.deepcopy(maze)
    res = bfs(copymaze,H,W,sj,si)
    max_dist = max(max_dist,res)

print(max_dist)