from collections import deque

h, w = map(int, input().split())

maze = [input() for i in range(h)]

ans = 0
INF = 20 * 20 + 1
for j in range(h):
    for i in range(w):
        if maze[j][i] == ".":
            cnt = [[INF] * w for _ in range(h)] 
            cnt[j][i] = 0
            q = deque([(j,i)])
            while q:
                y,x = q.popleft()
                for by,bx in ((0,1),(0,-1),(1,0),(-1,0)):
                    by += y
                    bx += x 
                    if 0 <= by < h and 0 <= bx < w:
                        ct = cnt[y][x] + 1
                        if maze[ny][nx] == "." and ct < cnt[ny][nx]:
                            q += [(by,bx)] 
                            cnt[by][bx] = ct
                            ans = max(ans, cnt[by][bx])

print(ans)