Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for dj, di in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                if ans < v+1:
                    ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj, di = d
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                if ans < v+1:
                    ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj, di = d
                if S[dj][di] == "." and m[dj][di] == -1:
                    q.append((dj, di, v+1))
                    if ans < v+1:
                        ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = []
    for _ in range(H):
        m.append([-1] * W)

    q = deque()
    q.append([y, x, 0])
    maximus = 0

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if dj < 0 or dj >= H or di < 0 or di >= W or m[dj][di] != -1 or S[dj][di] == "#":
                continue
            q.append([dj, di, v+1])
            if maximus < v+1:
                maximus = v+1

    return maximus


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        ans = max(ans, bfs(i, j))
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input() + "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj, di = d
                if S[dj][di] == "." and m[dj][di] == -1:
                    q.append((dj, di, v+1))
                    if ans < v+1:
                        ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input() + "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj, di = d
                if S[dj][di] == "." and m[dj][di] == -1:
                    q.append((dj, di, v+1))
                    if ans < v+1:
                        ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        m = [[-1] * W for _ in range(H)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj, di = d
                if 0 <= dj < H and 0 <= di < W and S[dj][di] == "." and m[dj][di] == -1:
                    q.append((dj, di, v+1))
                    if ans < v+1:
                        ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        m = [[-1] * W for _ in range(H)]

        q = deque()
        q.append((j, i, 0))

        while q:
            y, x, v = q.popleft()
            m[y][x] = v
            for d in ((y, x + 1), (y, x-1), (y+1, x), (y-1, x)):
                dj = d[0]
                di = d[1]
                if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                    q.append((dj, di, v+1))
                    if ans < v+1:
                        ans = v+1
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = [[-1] * W for _ in range(H)]

    q = deque()
    q.append([y, x, 0])
    global ans

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for d in ((j, i + 1), (j, i-1), (j+1, i), (j-1, i)):
            dj = d[0]
            di = d[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if ans < v+1:
                    ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = [[-1] * W for _ in range(H)]

    q = deque()
    q.append([y, x, 0])
    global ans

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if ans < v+1:
                    ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = [[-1] * W for _ in range(H)]

    q = deque()
    q.append([y, x, 0])
    maximus = 0

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if maximus < v+1:
                    maximus = v+1

    return maximus


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        ans = max(ans, bfs(i, j))
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0


def bfs(x, y):
    global ans
    m = []
    for _ in range(H):
        m.append([-1] * W)

    q = deque()
    q.append([y, x, 0])

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        dj = j + 1
        if dj < H and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        dj = j - 1
        if 0 <= dj and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        di = i + 1
        if di < W and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1
        di = i - 1
        if 0 <= di and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import numpy as np
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0


def bfs(x, y):
    global ans
    m = np.full((H, W), -1, dtype=np.int)

    q = deque()
    q.append([y, x, 0])

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        dj = j + 1
        if dj < H and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        dj = j - 1
        if 0 <= dj and m[dj][i] == -1 and S[dj][i] == ".":
            q.append([dj, i, v+1])
            if ans < v+1:
                ans = v+1
        di = i + 1
        if di < W and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1
        di = i - 1
        if 0 <= di and m[j][di] == -1 and S[j][di] == ".":
            q.append([j, di, v+1])
            if ans < v+1:
                ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import numpy as np
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ((0, 1), (0, -1), (1, 0), (-1, 0))


def bfs(x, y):
    global ans
    m = np.full((H, W), -1, dtype=np.int)

    q = deque()
    q.append([y, x, 0])

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if ans < v+1:
                    ans = v+1


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        bfs(i, j)
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import numpy as np
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = np.full((H, W), -1, dtype=np.int)

    q = deque()
    q.append([y, x, 0])
    maximus = 0

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if maximus < v+1:
                    maximus = v+1

    return maximus


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        ans = max(ans, bfs(i, j))
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Runtime Error):
import numpy as np
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = np.full((H, W), dtype=np.int)

    q = deque()
    q.append([y, x, 0])
    maximus = 0

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if maximus < v+1:
                    maximus = v+1

    return maximus


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        ans = max(ans, bfs(i, j))
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
import sys
input = sys.stdin.readline

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

ans = 0

d = ([0, 1], [0, -1], [1, 0], [-1, 0])


def bfs(x, y):
    m = []
    for _ in range(H):
        m.append([-1] * W)

    q = deque()
    q.append([y, x, 0])
    maximus = 0

    while q:
        j, i, v = q.popleft()
        m[j][i] = v
        for dd in d:
            dj = j + dd[0]
            di = i + dd[1]
            if 0 <= dj < H and 0 <= di < W and m[dj][di] == -1 and S[dj][di] == ".":
                q.append([dj, di, v+1])
                if maximus < v+1:
                    maximus = v+1

    return maximus


for j in range(H):
    for i in range(W):
        if S[j][i] != ".":
            continue
        ans = max(ans, bfs(i, j))
print(ans)


Fixed submission:
from collections import deque
import sys
input = sys.stdin.readline

H, W = list(map(int, input().split()))
S = ["#" * (W+2)] + ["#" + input().strip() +
                     "#" for _ in range(H)] + ["#" * (W+2)]

ans = 0

for j in range(1, H+1):
    for i in range(1, W+1):
        if S[j][i] != ".":
            continue
        m = [[-1] * (W+2) for _ in range(H+2)]

        q = deque()
        q.append((j, i, 0))
        m[j][i] = 0

        while q:
            y, x, v = q.popleft()
            if ans < v:
                ans = v
            for dj, di in ((y, x+1), (y, x-1), (y+1, x), (y-1, x)):
                if S[dj][di] == "#" or m[dj][di] != -1:
                    continue
                q.append((dj, di, v+1))
                m[dj][di] = v+1
print(ans)