# -*- coding: utf-8 -*-
from collections import deque
h,w = map(int, input().split())
s = [input() for _ in range(h)]

if h==1 and w==1:
    print(0)
    exit()

def solve(sh,sw,gh,gw):
    if s[sh][sw]=="#" or s[gh][gw]=="#":
        return -1

    visited = [[False for _ in range(w)] for _ in range(h)]
    visited[sh][sw] = True
    q = deque([(sh,sw,0)])
    while len(q)>0:
        hi,wi,r = q.popleft()
        for dh,dw in [[1,0],[0,1],[-1,0],[0,-1]]:
            if hi+dh>=0 and hi+dh<h and wi+dw>=0 and wi+dw<w and s[hi+dh][wi+dw]==".":
                if hi+dh==gh and wi+dw==gw:
                    return r+1
                elif not visited[hi+dh][wi+dw]:
                    visited[hi+dh][wi+dw] = True
                    q.append((hi+dh,wi+dw,r+1))

    return -1

res = -1
for shi in range(h):
    for swi in range(w):
        if s[shi][swi]=="#":
            continue
        for ghi in range(h-1,-1,-1):
            for gwi in range(w-1,-1,-1):
                if shi>=ghi and swi>=gwi:
                    break
                r = solve(shi,swi,ghi,gwi)
                res = max(res, r)

print(res)