Buggy python submission (verdict: Time Limit Exceeded):
import queue
import copy
q = queue.Queue()
h, w = map(int, input().split())
a = [list('#'+input()+'#') for _ in range(h)]
a = [['#']*(w+2)]+a+[['#']*(w+2)]
a1 = copy.deepcopy(a)
ans = []
        
for i in range(1, h+1):
    for j in range(1, w+1):
        a = copy.deepcopy(a1)
        if a[i][j] == '#':
            continue        
        q.put([i, j, 0])
        while not q.empty():
            now = q.get()
            if a[now[0]][now[1]] == '.':
                ans.append(now[2])
                a[now[0]][now[1]] = '#'
            if a[now[0]+1][now[1]] != '#':
                q.put([now[0]+1, now[1], now[2]+1])
            if a[now[0]-1][now[1]] != '#':
                q.put([now[0]-1, now[1], now[2]+1])
            if a[now[0]][now[1]+1] != '#':
                q.put([now[0], now[1]+1, now[2]+1])
            if a[now[0]][now[1]-1] != '#':
                q.put([now[0], now[1]-1, now[2]+1])
        a1[i][j] == '#'
print(max(ans))


Fixed submission:
import queue
import copy
q = queue.Queue()
h, w = map(int, input().split())
a = [list('#'+input()+'#') for _ in range(h)]
a = [['#']*(w+2)]+a+[['#']*(w+2)]
a1 = copy.deepcopy(a)
ans = []
        
for i in range(1, h+1):
    for j in range(1, w+1):
        a = copy.deepcopy(a1)
        if a[i][j] == '#':
            continue        
        q.put([i, j, 0])
        while not q.empty():
            now = q.get()
            
            if a[now[0]][now[1]] == '.':
                ans.append(now[2])
                a[now[0]][now[1]] = '#'
            else:
                continue
                
            if a[now[0]+1][now[1]] != '#':
                q.put([now[0]+1, now[1], now[2]+1])
            if a[now[0]-1][now[1]] != '#':
                q.put([now[0]-1, now[1], now[2]+1])
            if a[now[0]][now[1]+1] != '#':
                q.put([now[0], now[1]+1, now[2]+1])
            if a[now[0]][now[1]-1] != '#':
                q.put([now[0], now[1]-1, now[2]+1])
        a1[i][j] == '#'
print(max(ans))

Buggy python submission (verdict: Time Limit Exceeded):
import queue
import copy
q = queue.Queue()
h, w = map(int, input().split())
a = [list('#'+input()+'#') for _ in range(h)]
a = [['#']*(w+2)]+a+[['#']*(w+2)]
a1 = copy.deepcopy(a)
ans = []
        
for i in range(1, h+1):
    for j in range(1, w+1):
        a = copy.deepcopy(a1)
        if a[i][j] == '#':
            continue        
        q.put([i, j, 0])
        while not q.empty():
            now = q.get()
            if a[now[0]][now[1]] == '.':
                ans.append(now[2])
                a[now[0]][now[1]] = '#'
            if a[now[0]+1][now[1]] != '#':
                q.put([now[0]+1, now[1], now[2]+1])
            if a[now[0]-1][now[1]] != '#':
                q.put([now[0]-1, now[1], now[2]+1])
            if a[now[0]][now[1]+1] != '#':
                q.put([now[0], now[1]+1, now[2]+1])
            if a[now[0]][now[1]-1] != '#':
                q.put([now[0], now[1]-1, now[2]+1])
print(max(ans))


Fixed submission:
import queue
import copy
q = queue.Queue()
h, w = map(int, input().split())
a = [list('#'+input()+'#') for _ in range(h)]
a = [['#']*(w+2)]+a+[['#']*(w+2)]
a1 = copy.deepcopy(a)
ans = []
        
for i in range(1, h+1):
    for j in range(1, w+1):
        a = copy.deepcopy(a1)
        if a[i][j] == '#':
            continue        
        q.put([i, j, 0])
        while not q.empty():
            now = q.get()
            
            if a[now[0]][now[1]] == '.':
                ans.append(now[2])
                a[now[0]][now[1]] = '#'
            else:
                continue
                
            if a[now[0]+1][now[1]] != '#':
                q.put([now[0]+1, now[1], now[2]+1])
            if a[now[0]-1][now[1]] != '#':
                q.put([now[0]-1, now[1], now[2]+1])
            if a[now[0]][now[1]+1] != '#':
                q.put([now[0], now[1]+1, now[2]+1])
            if a[now[0]][now[1]-1] != '#':
                q.put([now[0], now[1]-1, now[2]+1])
        a1[i][j] == '#'
print(max(ans))