Buggy python submission (verdict: Runtime Error):
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        if maze[sy][sx] == '#':
            continue
        dist = [[-1] * W for _ in range(W)]
        for gy in range(H):
            for gx in range(W):
                if maze[gy][gx] == '#':
                    continue
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)



Fixed submission:
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        if maze[sy][sx] == '#':
            continue
        dist = [[-1] * W for _ in range(H)]
        for gy in range(H):
            for gx in range(W):
                if maze[gy][gx] == '#':
                    continue
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)

Buggy python submission (verdict: Runtime Error):
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        if maze[sy][sx] == '#':
            continue
        dist = [[-1] * W for _ in range(W)]
        for gy in range(H):
            for gx in range(W):
                if maze[gy][gx] == '#':
                    continue
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)



Fixed submission:
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        if maze[sy][sx] == '#':
            continue
        dist = [[-1] * W for _ in range(H)]
        for gy in range(H):
            for gx in range(W):
                if maze[gy][gx] == '#':
                    continue
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)

Buggy python submission (verdict: Runtime Error):
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        dist = [[-1] * W for _ in range(W)]
        for gy in range(H):
            for gx in range(W):
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)



Fixed submission:
H, W = map(int, input().split())
maze = [input() for _ in range(H)]


from collections import deque
def bfs(maze, dist, sy, sx):
    d = deque()
    d.append([sy, sx])
    while d:
        y, x = d.popleft()
        for dy, dx in [1, 0], [-1, 0], [0, 1], [0, -1]:
            ny = y + dy
            nx = x + dx
            if ny < 0 or ny >= H or nx < 0 or nx >= W:
                continue
            if maze[ny][nx] == '#':
                continue
            if dist[ny][nx] == -1:
                dist[ny][nx] = dist[y][x] + 1
                d.append([ny, nx])

ans = -1
for sy in range(H):
    for sx in range(W):
        if maze[sy][sx] == '#':
            continue
        dist = [[-1] * W for _ in range(H)]
        for gy in range(H):
            for gx in range(W):
                if maze[gy][gx] == '#':
                    continue
                dist[sy][sx] = 0
                bfs(maze, dist, sy, sx)
                ans = max(ans, dist[gy][gx])
print(ans)