Buggy python submission (verdict: Time Limit Exceeded):
import numpy as np
def min_rounte(s, sta, path):
    move = [[0,1],[0,-1],[1,0],[-1,0]]
    for i in move:
        ax = [sta[0]+i[0], sta[1]+i[1]]
        if s[ax[0]][ax[1]] == -1:
            continue
        if s[ax[0]][ax[1]] > path:
            s[ax[0]][ax[1]] = path
    return s
 
def bfs(s, sta):
    s_ = np.array(s)
    s_[sta[0]][sta[1]] = 0
    for i in range(400):
        ind = np.where(s_==i)
        for j in range(len(ind[0])):
            s_ = min_rounte(s_, [ind[0][j], ind[1][j]], i+1)
        if len(ind[0]) == 0:
            break
    return i-1
        

h, w = map(int, input().split())
#s_ = [[0 if i=="." else -1 for i in input()] for j in range(h)]
s = [[-1 for i in range(w+2)] for j in range(h+2)]
for i in range(h):
    a = input()
    for j in range(w):
        if a[j] == ".":
            s[i+1][j+1] = 512
        else:
            s[i+1][j+1] = -1
  
ans = 1

for i in range(1, h+1):
    for j in range(1, w+1):
        if s[i][j] == -1:
            continue
        temp = bfs(s, [i,j])
        if temp > ans:
            ans = temp
print(ans)

Fixed submission:
import numpy as np
def min_rounte(s, sta, path):
    move = [[0,1],[0,-1],[1,0],[-1,0]]
    for i in move:
        ax = [sta[0]+i[0], sta[1]+i[1]]
        if s[ax[0]][ax[1]] == -1:
            continue
        if s[ax[0]][ax[1]] > path:
            s[ax[0]][ax[1]] = path
    return s
 
def bfs(s, sta):
    s_ = np.array(s)
    s_[sta[0]][sta[1]] = 0
    tmp = sta
    for i in range(400):
        ind = np.where(s_==i)
        for j in range(len(ind[0])):
            s_ = min_rounte(s_, [ind[0][j], ind[1][j]], i+1)
        if len(ind[0]) == 0:
            break
        tmp = [[ind[0][k], ind[1][k]] for k in range(len(ind[0]))]
    return i-1, tmp
        

h, w = map(int, input().split())
#s_ = [[0 if i=="." else -1 for i in input()] for j in range(h)]
s = [[-1 for i in range(w+2)] for j in range(h+2)]
for i in range(h):
    a = input()
    for j in range(w):
        if a[j] == ".":
            s[i+1][j+1] = 512
        else:
            s[i+1][j+1] = -1

for i in range(1, h+1):
    for j in range(1, w+1):
        if s[i][j] == -1:
            continue
        temp, ax = bfs(s, [i,j])
        break
ans = 1
for i in ax:
    ans_, t = bfs(s, i)
    if ans_ > ans:
        ans = ans_

print(ans)

Buggy python submission (verdict: Runtime Error):
import numpy as np
def min_rounte(s, sta, path):
    move = [[0,1],[0,-1],[1,0],[-1,0]]
    for i in move:
        ax = [sta[0]+i[0], sta[1]+i[1]]
        if s[ax[0]][ax[1]] == -1:
            continue
        if s[ax[0]][ax[1]] > path:
            s[ax[0]][ax[1]] = path
    return s
 
def bfs(s, sta):
    s_ = np.array(s)
    s_[sta[0]][sta[1]] = 0
    for i in range(400):
        ind = np.where(s_==i)
        for j in range(len(ind[0])):
            s_ = min_rounte(s_, [ind[0][j], ind[1][j]], i+1)
        if len(ind[0]) == 0:
            break
    return i-1
        

h, w = map(int, input().split())
#s_ = [[0 if i=="." else -1 for i in input()] for j in range(h)]
s = [[-1 for i in range(w+2)] for j in range(h+2)]
for i in range(h):
    a = input()
    for j in range(w):
        if a[j] == ".":
            s[i+1][j+1] = 512
        else:
            s[i+1][j+1] = -1
  
ans = 1

for i in range(1, h+1):
    for j in range(1, w+1):
        if s[i][j] == -1:
            continue
        temp = bfs(s, [i,j])
        if temp > ans:
            ans = temp
print(ans)

Fixed submission:
import numpy as np
def min_rounte(s, sta, path):
    move = [[0,1],[0,-1],[1,0],[-1,0]]
    for i in move:
        ax = [sta[0]+i[0], sta[1]+i[1]]
        if s[ax[0]][ax[1]] == -1:
            continue
        if s[ax[0]][ax[1]] > path:
            s[ax[0]][ax[1]] = path
    return s
 
def bfs(s, sta):
    s_ = np.array(s)
    s_[sta[0]][sta[1]] = 0
    tmp = sta
    for i in range(400):
        ind = np.where(s_==i)
        for j in range(len(ind[0])):
            s_ = min_rounte(s_, [ind[0][j], ind[1][j]], i+1)
        if len(ind[0]) == 0:
            break
        tmp = [[ind[0][k], ind[1][k]] for k in range(len(ind[0]))]
    return i-1, tmp
        

h, w = map(int, input().split())
#s_ = [[0 if i=="." else -1 for i in input()] for j in range(h)]
s = [[-1 for i in range(w+2)] for j in range(h+2)]
for i in range(h):
    a = input()
    for j in range(w):
        if a[j] == ".":
            s[i+1][j+1] = 512
        else:
            s[i+1][j+1] = -1

for i in range(1, h+1):
    for j in range(1, w+1):
        if s[i][j] == -1:
            continue
        temp, ax = bfs(s, [i,j])
        break
ans = 1
for i in ax:
    ans_, t = bfs(s, i)
    if ans_ > ans:
        ans = ans_

print(ans)