Buggy python submission (verdict: Runtime Error):
import copy
import sys
sys.setrecursionlimit(10**8)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0) ,walker)
    else:
        return None


candicate = []
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
        far=0
        for gx in range(W):
            for gy in range(H):
                if S[gy][gx]=='.':
                    if far <= temp_walker[gy][gx]:
                        far = temp_walker[gy][gx]

        #print(sx, sy, temp_walker, far)
        candicate.append(far)
print(max(candicate))




Fixed submission:
import copy
import sys
sys.setrecursionlimit(10**6)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0) ,walker)
    else:
        return None


candicate = []
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
            far=0

            for gx in range(W):
                for gy in range(H):
                    if S[gy][gx]=='.':
                        if far < temp_walker[gy][gx]:
                            far = temp_walker[gy][gx]

            #print(sx, sy, temp_walker, far)
            candicate.append(far)
print(max(candicate))

Buggy python submission (verdict: Runtime Error):
import copy
import sys
sys.setrecursionlimit(10**6)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0),walker)
    else:
        return None


candicate=[]
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
        far=0
        for gx in range(W):
            for gy in range(H):
                if S[gy][gx]=='.':
                    if far <= temp_walker[gy][gx]:
                        far = temp_walker[gy][gx]

        #print(sx, sy, temp_walker, far)
        candicate.append(far)
print(max(candicate))




Fixed submission:
import copy
import sys
sys.setrecursionlimit(10**6)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0) ,walker)
    else:
        return None


candicate = []
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
            far=0

            for gx in range(W):
                for gy in range(H):
                    if S[gy][gx]=='.':
                        if far < temp_walker[gy][gx]:
                            far = temp_walker[gy][gx]

            #print(sx, sy, temp_walker, far)
            candicate.append(far)
print(max(candicate))

Buggy python submission (verdict: Runtime Error):
import copy


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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0),walker)
    else:
        return None


candicate=[]
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
        far=0
        for gx in range(W):
            for gy in range(H):
                if S[gy][gx]=='.':
                    if far <= temp_walker[gy][gx]:
                        far = temp_walker[gy][gx]

        #print(sx, sy, temp_walker, far)
        candicate.append(far)
print(max(candicate))




Fixed submission:
import copy
import sys
sys.setrecursionlimit(10**6)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0) ,walker)
    else:
        return None


candicate = []
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
            far=0

            for gx in range(W):
                for gy in range(H):
                    if S[gy][gx]=='.':
                        if far < temp_walker[gy][gx]:
                            far = temp_walker[gy][gx]

            #print(sx, sy, temp_walker, far)
            candicate.append(far)
print(max(candicate))

Buggy python submission (verdict: Runtime Error):
import copy
import sys
sys.setrecursionlimit(400)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0),walker)
    else:
        return None


candicate=[]
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
        far=0
        for gx in range(W):
            for gy in range(H):
                if S[gy][gx]=='.':
                    if far <= temp_walker[gy][gx]:
                        far = temp_walker[gy][gx]

        #print(sx, sy, temp_walker, far)
        candicate.append(far)
print(max(candicate))




Fixed submission:
import copy
import sys
sys.setrecursionlimit(10**6)

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

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

walker=[]
for row in range(H):
    walker.append([None]*W)
waiting=[]
def func(xy, walker):
    x, y = xy
    for i in range(4):
        if 0 <= x+dx[i] < W and 0 <= y+dy[i] <H:
            if walker[y+dy[i]][x+dx[i]] == None and S[y+dy[i]][x+dx[i]] == '.':
                walker[y+dy[i]][x+dx[i]] = walker[y][x]+1
                waiting.append((x+dx[i], y+dy[i]))
    if waiting:
        func(waiting.pop(0) ,walker)
    else:
        return None


candicate = []
for sx in range(W):
    for sy in range(H):
        if S[sy][sx] == '.':
            temp_walker=copy.deepcopy(walker)
            temp_walker[sy][sx]=0
            func((sx,sy), temp_walker)
            far=0

            for gx in range(W):
                for gy in range(H):
                    if S[gy][gx]=='.':
                        if far < temp_walker[gy][gx]:
                            far = temp_walker[gy][gx]

            #print(sx, sy, temp_walker, far)
            candicate.append(far)
print(max(candicate))