from collections import deque
#import numpy as np

h, w = map(int, input().split())
S = [input() for _ in range(h)] # #.のグリッドの場合

starts = []
for i in range(h):
    for j in range(w):
        if S[i][j] == '.':
            starts.append((i,j))

step = [(1, 0), (-1, 0), (0, 1), (0, -1)]
ans = 0
for si in starts:
    maxlen = 0
    seen = [[0] * w for _ in range(h)]
    que = deque()
    que.append(si)
    while len(que) > 0:
        x, y = que.popleft()
        for xs, ys in step:
            xi, yi = x + xs, y + ys
            if xi < 0 or yi < 0 or xi >= h or yi >= w:
                continue
            if S[xi][yi] == '#':
                continue
            if seen[xi][yi] != 0:
                continue
            else:
                seen[xi][yi] = seen[x][y] + 1
                que.append((xi, yi))
                maxlen = max(maxlen, seen[xi][yi])
    #print(np.array(seen))
    ans = max(ans, maxlen)
print(ans)