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

read = sys.stdin.read
readline = sys.stdin.readline

H, W = map(int, readline().split())
S = tuple(read().split())


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)


Fixed submission:
import sys
from collections import deque

read = sys.stdin.read
readline = sys.stdin.readline

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


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        if visited[x][y]:
            continue
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)

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

read = sys.stdin.read
readline = sys.stdin.readline

H, W = map(int, readline().split())
S = tuple(read().split())


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        visited[x][y]=True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp

print(answer)


Fixed submission:
import sys
from collections import deque

read = sys.stdin.read
readline = sys.stdin.readline

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


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        if visited[x][y]:
            continue
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)

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

read = sys.stdin.read
readline = sys.stdin.readline

H, W = map(int, readline().split())
S = tuple(read().split())


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = set()
    while queue:
        x, y = queue.popleft()
        visited.add((x, y))

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and (new_x, new_y) not in visited:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp

print(answer)


Fixed submission:
import sys
from collections import deque

read = sys.stdin.read
readline = sys.stdin.readline

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


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        if visited[x][y]:
            continue
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)

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

read = sys.stdin.read
readline = sys.stdin.readline

H, W = map(int, readline().split())
S = tuple(read().split())


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = set()
    while queue:
        x, y = queue.popleft()
        visited.add((x, y))

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and (new_x, new_y) not in visited:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue
        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp

print(answer)


Fixed submission:
import sys
from collections import deque

read = sys.stdin.read
readline = sys.stdin.readline

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


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        if visited[x][y]:
            continue
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)

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

read = sys.stdin.read
readline = sys.stdin.readline

H, W = map(int, readline().split())
S = tuple(read().split())


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = set()
    while queue:
        x, y = queue.popleft()
        visited.add((x, y))

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and (new_x, new_y) not in visited:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue
        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp

print(answer)


Fixed submission:
import sys
from collections import deque

read = sys.stdin.read
readline = sys.stdin.readline

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


def bfs(grid, start):
    distance = [[0] * W for _ in range(H)]
    queue = deque([start])
    visited = [[False] * W for _ in range(H)]
    while queue:
        x, y = queue.popleft()
        if visited[x][y]:
            continue
        visited[x][y] = True

        for i, j in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and not visited[new_x][new_y]:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return distance


answer = 0
limit = H - 1 + W - 1
for i in range(H):
    for j in range(W):
        if S[i][j] == '#':
            continue

        cnt = 0
        for dx, dy in ([1, 0], [-1, 0], [0, 1], [0, -1]):
            new_y, new_x = dy + j, dx + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if S[new_x][new_y] == '.':
                    cnt += 1

        if cnt > 2:
            continue

        tmp = max(map(max, bfs(S, (i, j))))
        if tmp > answer:
            answer = tmp
        if answer == limit:
            print(answer)
            exit()

print(answer)