Buggy python submission (verdict: Time Limit Exceeded):
import copy
import collections
import sys


def input():
    return sys.stdin.readline()[:-1]


def bfs(sx, sy, gx, gy):
    # sx,sy := スタートの座標
    # gx,gy := ゴールの座標
    que = collections.deque()
    que.append([sx, sy])
    d = [[inf]*W for _ in range(H)]
    d[sx][sy] = 0
    s = copy.copy(field)
    while (len(que) > 0):
        p = que.popleft()
        if p[0] == gx and p[1] == gy:
            break
        for i in range(4):
            nx = p[0] + dx[i]
            ny = p[1] + dy[i]
            if (0 <= nx and nx < H and 0 <= ny and ny < W and s[nx][ny] != "#" and d[nx][ny] == inf):
                que.append([nx, ny])
                d[nx][ny] = d[p[0]][p[1]] + 1
    return d[gx][gy]


inf = float('inf')
H, W = map(int, input().split())
field = [list(input()) for _ in range(H)]
d = [[inf]*W for _ in range(H)]

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

cand = []
memo = []

for h1 in range(H):
    for w1 in range(W):
        if field[h1][w1] == '#':
            continue
        for h2 in range(H):
            for w2 in range(W):
                if field[h2][w2] == '#':
                    continue
                if h1 == h2 and w1 == w2:
                    continue
                cand.append([h1, w1, h2, w2])
for c in cand:
    res = bfs(*c)
    if res != float('inf'):
        memo.append(res)
print(max(memo))


Fixed submission:
import copy
import collections
import sys


def input():
    return sys.stdin.readline()[:-1]


def bfs(sx, sy):
    # sx,sy := スタートの座標
    # gx,gy := ゴールの座標
    que = collections.deque()
    que.append([sx, sy])
    d = [[inf]*W for _ in range(H)]
    d[sx][sy] = 0
    s = copy.copy(field)
    while (len(que) > 0):
        p = que.popleft()
        for i in range(4):
            nx = p[0] + dx[i]
            ny = p[1] + dy[i]
            if (0 <= nx and nx < H and 0 <= ny and ny < W and s[nx][ny] != "#" and d[nx][ny] == inf):
                que.append([nx, ny])
                d[nx][ny] = d[p[0]][p[1]] + 1
    return d  # d[gx][gy]


inf = -1
H, W = map(int, input().split())
field = [list(input()) for _ in range(H)]
d = [[inf]*W for _ in range(H)]

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

cand = []
memo = []

for h1 in range(H):
    for w1 in range(W):
        if field[h1][w1] == '#':
            continue
        cand.append([h1, w1])
for c in cand:
    res = bfs(*c)
    memo.append(max(sum(res, [])))
print(max(memo))

Buggy python submission (verdict: Time Limit Exceeded):
import copy
import collections
import sys


def input():
    return sys.stdin.readline()[:-1]


def bfs(sx, sy, gx, gy):
    # sx,sy := スタートの座標
    # gx,gy := ゴールの座標
    que = collections.deque()
    que.append([sx, sy])
    d = [[inf]*W for _ in range(H)]
    d[sx][sy] = 0
    s = copy.copy(field)
    while (len(que) > 0):
        p = que.popleft()
        if p[0] == gx and p[1] == gy:
            break
        for i in range(4):
            nx = p[0] + dx[i]
            ny = p[1] + dy[i]
            if (0 <= nx and nx < H and 0 <= ny and ny < W and s[nx][ny] != "#" and d[nx][ny] == inf):
                que.append([nx, ny])
                d[nx][ny] = d[p[0]][p[1]] + 1
    return d[gx][gy]


inf = float('inf')
H, W = map(int, input().split())
field = [list(input()) for _ in range(H)]
d = [[inf]*W for _ in range(H)]

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

cand = []
memo = []

for h1 in range(H):
    for w1 in range(W):
        for h2 in range(H):
            for w2 in range(W):
                if h1 == h2 and w1 == w2:
                    continue
                cand.append([h1, w1, h2, w2])
for c in cand:
    res = bfs(*c)
    if res != float('inf'):
        memo.append(res)
print(max(memo))


Fixed submission:
import copy
import collections
import sys


def input():
    return sys.stdin.readline()[:-1]


def bfs(sx, sy):
    # sx,sy := スタートの座標
    # gx,gy := ゴールの座標
    que = collections.deque()
    que.append([sx, sy])
    d = [[inf]*W for _ in range(H)]
    d[sx][sy] = 0
    s = copy.copy(field)
    while (len(que) > 0):
        p = que.popleft()
        for i in range(4):
            nx = p[0] + dx[i]
            ny = p[1] + dy[i]
            if (0 <= nx and nx < H and 0 <= ny and ny < W and s[nx][ny] != "#" and d[nx][ny] == inf):
                que.append([nx, ny])
                d[nx][ny] = d[p[0]][p[1]] + 1
    return d  # d[gx][gy]


inf = -1
H, W = map(int, input().split())
field = [list(input()) for _ in range(H)]
d = [[inf]*W for _ in range(H)]

dx = [1, 0, -1, 0]
dy = [0, 1, 0, -1]

cand = []
memo = []

for h1 in range(H):
    for w1 in range(W):
        if field[h1][w1] == '#':
            continue
        cand.append([h1, w1])
for c in cand:
    res = bfs(*c)
    memo.append(max(sum(res, [])))
print(max(memo))