from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = []
for _ in range(H):
    S.append(input())

ans = 0


def bfs(x, y):
    global ans
    m = []
    for _ in range(H):
        m.append([-1] * W)

    q = deque()
    q.append([y, x, 0])

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        dj = j + 1
        if dj < H and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        dj = j - 1
        if 0 <= dj and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        di = i + 1
        if di < W and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1
        di = i - 1
        if 0 <= di and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)
