from collections import Counter,defaultdict,deque
import sys
import bisect
import math
import itertools
import string
import queue
import copy
# import numpy as np
# import scipy
from itertools import permutations, combinations
from heapq import heappop, heappush
# input = sys.stdin.readline
sys.setrecursionlimit(10**8)
mod = 10**9+7
 
def inp(): # n=1
    return int(input())
def inpm(): # x=1,y=2
    return map(int,input().split())
def inpl(): # a=[1,2,3,4,5,...,n]
    return list(map(int, input().split()))
def inpls(): # a=['1','2','3',...,'n']
    return list(input().split())
def inplm(n): # x=[] 複数行
    return list(int(input()) for _ in range(n))
def inpll(n): # [[1,1,1,1],[2,2,2,2],[3,3,3,3]]
    return [list(map(int, input().split())) for _ in range(n)]


def main():
    h,w = inpm()
    maze = []
    for _ in range(h):
        s = list(input())
        maze.append(s)
    ans = 0
    dx=[1,0,-1,0]
    dy=[0,1,0,-1]
    for sx in range(h):
        for sy in range(w):
            if maze[sx][sy] == '#':
                continue
            else:
                for gx in range(h):
                    for gy in range(w):
                        if sx==gx and sy==gy:
                            continue
                        else:
                            d = [[-1 for _ in range(w)] for _ in range(h)]
                            que = []
                            d[sx][sy] = 0
                            que.append([sx,sy])
                            while que:
                                [qx,qy]=que[0]
                                if qx==gx and qy==gy:
                                    break
                                que.pop(0)
                                for i in range(4):
                                    nx=qx+dx[i]
                                    ny=qy+dy[i]
                                    if nx>=0 and h>nx and ny>=0 and w>ny and maze[nx][ny]=="." and d[nx][ny]==-1:
                                        d[nx][ny]=d[qx][qy]+1
                                        que.append([nx,ny])
                            ans = max(ans,d[gx][gy])
    print(ans)



if __name__ == "__main__":
    main()