import sys
import math
import heapq
from collections import deque
from itertools import combinations
sys.setrecursionlimit(10**7)
INTMAX = 9223372036854775807
INTMIN = -9223372036854775808
DVSR = 1000000007
def POW(x, y): return pow(x, y, DVSR)
def INV(x, m=DVSR): return pow(x, m - 2, m)
def DIV(x, y, m=DVSR): return (x * INV(y, m)) % m
def LI(): return [int(x) for x in input().split()]
def LF(): return [float(x) for x in input().split()]
def LS(): return input().split()
def II(): return int(input())
def FLIST(n):
    res = [1]
    for i in range(1, n+1): res.append(res[i-1]*i%DVSR)
    return res
def gcd(x, y):
    if x < y: x, y = y, x
    div = x % y
    while div != 0:
        x, y = y, div
        div = x % y
    return y

H,W=LI()
MP=[input() for _ in range(H)]
CORD=[(i,j) for i in range(H) for j in range(W)]

max_dist = 0
visited=set()
for i in range(len(CORD)):
    for j in range(i+1, len(CORD)):
        st, en = CORD[i], CORD[j]
        if MP[st[0]][st[1]] == "." and MP[en[0]][en[1]] == ".":
            QUE=deque()
            visited.add(st)
            QUE.append((st[0], st[1], 0))
            res = 0
            while QUE:
                ii, jj, dist = QUE.popleft()
                if (ii, jj) == en:
                    res = dist
                    visited.clear()
                    break
                iii, jjj = ii-1, jj
                if iii >= 0 and MP[iii][jjj] != '#' and not (iii, jjj) in visited:
                    QUE.append((iii, jjj, dist+1))
                    visited.add((iii, jjj))

                iii, jjj = ii+1, jj
                if iii < H and MP[iii][jjj] != '#' and not (iii, jjj) in visited:
                    QUE.append((iii, jjj, dist+1))
                    visited.add((iii, jjj))

                iii, jjj = ii, jj-1
                if jjj >= 0 and MP[iii][jjj] != '#' and not (iii, jjj) in visited:
                    QUE.append((iii, jjj, dist+1))
                    visited.add((iii, jjj))
                iii, jjj = ii, jj+1
                if jjj < W and MP[iii][jjj] != '#' and not (iii, jjj) in visited:
                    QUE.append((iii, jjj, dist+1))
                    visited.add((iii, jjj))


            max_dist = max(res, max_dist)

print(max_dist)
