from collections import deque

H, W = map(int, input().split())
m = [list(input()) for i in range(H)]
ans = 0

for h in range(H):
  for w in range(W):
    if m[h][w] == "#": continue
    q = deque([(h, w, 0)])
    used = set()
    while q:
      h, w, c = q.popleft()
      if (h, w) in used: continue
      ans = max(ans, c)
      if 0 <= h-1 and m[h-1][w] == "." and (h-1, w) not in used: q.append((h-1, w, c+1))
      if h+1 < H  and m[h+1][w] == "." and (h+1, w) not in used: q.append((h+1, w, c+1))
      if 0 <= w-1 and m[h][w-1] == "." and (h, w-1) not in used: q.append((h, w-1, c+1))
      if w+1 < W  and m[h][w+1] == "." and (h, w+1) not in used: q.append((h, w+1, c+1))
      used.add((h, w))
print(ans)