from collections import deque

def bfs(maze, h, w, si, sj, gi, gj):
    memo = [[0] * w for i in range(h)]
    q = deque([[si, sj]])
    score = [[0] * w for i in range(h)]

    pattern = [[1, 0], [-1, 0], [0, 1], [0, -1]]
    while len(q) > 0:
        i, j = q.popleft()
        if i == gi and j == gj:
            return score[gi][gj]
        for move in pattern:
            ni, nj= i + move[0], j + move[1]
            if ni < 0 or h <= ni or nj < 0 or w <= nj:
                continue
            p = maze[ni][nj]
            if p == '#':
                continue
            if p == '.' and not score[ni][nj]:
                score[ni][nj] = score[i][j] + 1
                q.append([ni, nj])

h, w = map(int, input().split())
maze = [input() for i in range(h)]
memo = []
scores = []
for si in range(h):
    for sj in range(w):
        if maze[si][sj] == "#":
            continue
        for gi in range(h):
            for gj in range(w):
                if maze[gi][gj] == "#" or (si == gi and sj == gj) or (gi, gj) in memo:
                    continue
                scores.append(bfs(maze, h, w, si, sj, gi, gj))
print(max(scores))