from collections import deque

H, W = map(int, input().split())

S = [['#'] * (W + 2)]
for _ in range(H):
    tmp = ['#'] + list(input()) + ['#']
    S.append(tmp)
S.append(['#'] * (W + 2))

move = [[1, 0], [-1, 0], [0, 1], [0, -1]]

ans = 0
for X in range(1, H + 1):
    for Y in range(1, W + 1):
        check = [[-1] * (W + 2) for _ in range(H + 2)]
        check[X][Y] = 0
        que = deque()
        que.append([X, Y])
        while que:
            tmp = que.popleft()
            x = tmp[0]
            y = tmp[1]
            flag = True
            for dx, dy in move:
                nx = x + dx
                ny = y + dy
                if S[nx][ny] == '.' and check[nx][ny] == -1:
                    que.append([nx, ny])
                    check[nx][ny] = check[x][y] + 1
                    flag = False
            if flag:
                ans = max(ans, check[x][y])


print (ans)