from collections import deque
from copy import copy

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

dxs = [0, 0, 1, -1]
dys = [1, -1, 0, 0]
cand = []
field_org = [-1] * (W * H)
for j in range(H):
    for i in range(W):
        if S[j][i] == '#':
            continue
        q = deque()
        q.append((i, j, 0))
        field = copy(field_org)
        field[j * H + i] = 0
        while q:
            p = q.popleft()
            for dx, dy in zip(dxs, dys):
                x, y = p[0] + dx, p[1] + dy
                f_idx = y * H + x
                if not (0 <= x < W) or not (0 <= y < H) or S[y][x] == '#' or field[f_idx] != -1:
                    continue
                field[f_idx] = p[2] + 1
                q.append((x, y, p[2] + 1))
        cand.append(max(field))
print(max(cand))