from collections import deque
''' 
INF = float('inf')
MOD = 10 ** 9 + 7
'''
## ここは１つにまとめれる
# dy = (0, -1, 0, 1)
# dx = (1, 0, -1, 0)
d=[(1,0),(0,1),(-1,0),(0,-1)]

    
H,W=map(int,input().split())
M = []
Max_list = []
for i  in range(H):
    t = input()
    M.append(t)
dist = [[-1]*W for _ in range(H)]
q = deque()
for i in range(H):
    for j in range(W):
        dist = [[-1]*W for _ in range(H)]
        if M[i][j] == '.':
            Ma = 0
            q.append((i, j))
            dist[i][j] = 0
            while q:
                cx,cy = q.popleft()
                for nx,ny in d:
                    x ,y= cx+nx,cy+ny
                    ''' ここの処理がをnotにしてわかりやすく
                    if x<0 or y<0 or x>=H or y>=W or dist[x][y]>0:
                        continue
                    '''
                    '''
                    if 0<= x < H and 0<= y < W and dist[x][y]<=0:
                    '''
                    if 0<= x < H and 0<= y < W and M[x][y] == ".":
                        dist[x][y]=dist[cx][cy]+1
                        Ma = max(Ma,dist[x][y])
                        q.append((x,y))
            Max_list.append(Ma)

print(max(Max_list))