import queue
import sys

input = sys.stdin.readline


class Solution:
    def main(self):
        H, W = map(int, input().split())
        S = [list(input().rstrip()) for _ in range(H)]

        for i in range(H):
            S[i] = ["#"] + S[i] + ["#"]
        S = [["#"] * (W + 2)] + S + [["#"] * (W + 2)]

        already = set()
        ans = 0
        for i in range(1, H + 1):
            for j in range(1, W + 1):
                if S[i][j] == "#":
                    continue
                for n in range(1, H + 1):
                    for m in range(1, W + 1):
                        if S[n][m] == "#" or (i, j) == (n, m) or (i, j, n, m) in already:
                            continue
                        v = self.min_move(S, (i, j), (n, m))
                        already.add((i, j, n, m))
                        already.add((n, m, i, j))
                        ans = max(ans, v)
        print(ans)

    def min_move(self, map, start, goal):
        moving_range = ((1, 0), (-1, 0), (0, 1), (0, -1))

        q = queue.Queue()
        q.put((start, 0))

        while not q.empty():
            pos, cnt = q.get()
            for move in moving_range:
                i, j = pos[0] + move[0], pos[1] + move[1]
                if (i, j) == goal:
                    return cnt + 1
                if map[i][j] == ".":
                    q.put(((i, j), cnt + 1))


# Run main
if __name__ == '__main__':
    Solution().main()
