#input
H, W = map(int, input().split())
import numpy as np
S = np.array([input() for _ in range(H)])

#output
import numpy as np
from collections import deque
INF = -1
dx = [1, 0, -1, 0]
dy = [0, 1, 0 ,-1]

def clear_maze(sx, sy, gx, gy, maze):
    def bfs():
        q = []
        q.insert(0, (sx, sy))
        dp[sx, sy] = 0

        while len(q):
            x, y = q.pop()

            if x == gx and y == gy:
                break

            for i in range(0, 4):
                nx, ny = x + dx[i], y + dy[i]

                if 0 <= nx < H and 0 <= ny < W and maze[nx][ny] != "#" and dp[nx, ny] == INF:
                    q.insert(0, (nx, ny))
                    dp[nx, ny] = dp[x, y] + 1
        return dp[gx, gy]
    return bfs()

answer = 0
for i in range(H):
    for j in range(W):
        if S[i][j] != "#":
            dp = np.full((H, W), INF)
            q = []
            q.insert(0, (i, j))
            dp[i, j] = 0
            while len(q):
                x, y = q.pop()

                for t in range(4):
                    nx, ny = x + dx[t], y + dy[t]

                    if (0 <= nx < H) and (0 <= ny < W) and (S[nx][ny] != "#") and (dp[nx, ny] == INF):
                        q.insert(0, (nx, ny))
                        dp[nx, ny] = dp[x, y] + 1
            for row in dp:
                answer = max(answer, max(row))
print(int(answer))