h,w = map(int,input().split())
meiro = [list(input()) for i in range(h)]
max_d = 0
from collections import deque

def bfs(sx,sy):
    que = deque([(sx,sy,0)])
    d = [[0]*w for i in range(h)]
    d[sx][sy] = 1
    max_len=0
    while que:
        x,y,tmp_len = que.popleft()
        for dx,dy in [(0,1),(1,0),(-1,0),(0,-1)]:
            tmp = tmp_len
            nx,ny = x+dx,y+dy
            if 0 <= nx < h and 0 <= ny < w and d[nx][ny] == 0 and meiro[nx][ny] =='.':
                d[nx][ny] = 1
                max_len = max(max_len, tmp+1)
                que.append((nx,ny,tmp+1))
    return max_len

for s_i in range(h):
    for s_j in range(w):
        max_d = max(bfs(s_i,s_j), max_d)
print(max_d)