### BUGGY CODE ###

import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from itertools import accumulate, permutations, combinations, product
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

H, W = MAP()
maze = [list(input()) for _ in range(H)]
dp = [[INF]*W*H for _ in range(W*H)]

for i in range(H*W):
	for j in range(H*W):
		if maze[i//W][i%W] == "#":
			continue
		if maze[j//W][j%W] == "#":
			continue
		if i == j:
			dp[i][j] = 0
		if i + W == j:
			dp[i][j] = 1
		if i + 1 == j:
			if i%W == W - 1:
				continue
			dp[i][j] = 1
		if i - W == j:
			dp[i][j] = 1
		if i - 1 == j:
			if i%W == 0:
				continue
			dp[i][j] = 1

# for i in range(W*H):
# 	for s in range(W*H):
# 		for e in range(W*H):
# 			# if maze[i//W][i%W] == "#":
# 			# 	continue
# 			# if maze[s//W][s%W] == "#":
# 			# 	continue
# 			# if maze[e//W][e%W] == "#":
# 			# 	continue
# 			dp[s][e] = min(dp[s][e], dp[s][i] + dp[i][e]);

for i in range(W*H):
    for j, c2 in enumerate(row[i] for row in dp):
           for k, (c1, c3) in enumerate(zip(dp[j], dp[i])):
               if c1 > c2+c3:
                   dp[j][k] = c2+c3

ma = 0
for i in range(len(dp)):
	for j in range(len(dp[i])):
		if dp[i][j] == INF:
			continue
		ma = max(ma, dp[i][j])

print(int(ma))

### FIXED CODE ###

import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from itertools import accumulate, permutations, combinations, product
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

def main():
	H, W = MAP()
	maze = [list(input()) for _ in range(H)]
	dp = [[INF]*W*H for _ in range(W*H)]

	for i in range(H*W):
		for j in range(H*W):
			if maze[i//W][i%W] == "#":
				continue
			if maze[j//W][j%W] == "#":
				continue
			if i == j:
				dp[i][j] = 0
			if i + W == j:
				dp[i][j] = 1
			if i + 1 == j:
				if i%W == W - 1:
					continue
				dp[i][j] = 1
			if i - W == j:
				dp[i][j] = 1
			if i - 1 == j:
				if i%W == 0:
					continue
				dp[i][j] = 1

	for i in range(W*H):
	    for j, c2 in enumerate(row[i] for row in dp):
	           for k, (c1, c3) in enumerate(zip(dp[j], dp[i])):
	               if c1 > c2+c3:
	                   dp[j][k] = c2+c3

	ma = 0
	for i in range(len(dp)):
		for j in range(len(dp[i])):
			if dp[i][j] == INF:
				continue
			ma = ma if ma > dp[i][j] else dp[i][j]

	print(int(ma))

if __name__ == "__main__":
	main()