Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from collections import deque
import math


def inputIntList():
  return [int(s) for s in input().split()]


def inputInt():
  return int(input())


def main():
  H, W = inputIntList()
  S = [[s for s in input()] for _ in range(H)]

  ans = 0
  for si in range(H):
    for sj in range(W):
      if S[si][sj] == '#':
        continue

      dist = [[-1 for _ in range(W)] for _ in range(H)]
      q = deque()

      def update(pos, num):
        i, j = pos
        if dist[i][j] != -1:
          return
        dist[i][j] = num
        q.append(pos)
        ans = max(ans, num)

      start = (si, sj)
      update(start, 0)

      while q:
        qi, qj = q.popleft()
        for di, dj in (-1, 0), (0, -1), (1, 0), (0, 1):
          ni = qi + di
          nj = qj + dj
          if ni < 0 or ni >= H:
            continue
          if nj < 0 or nj >= W:
            continue
          if S[ni][nj] == '#':
            continue
          update((ni, nj), dist[qi][qj]+1)

    return ans


if __name__ == "__main__":
  print(main())


Fixed submission:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from collections import deque
import math


def inputIntList():
  return [int(s) for s in input().split()]


def inputInt():
  return int(input())


def main():
  H, W = inputIntList()
  S = [[s for s in input()] for _ in range(H)]

  ans = 0
  for si in range(H):
    for sj in range(W):
      if S[si][sj] == '#':
        continue

      dist = [[-1 for _ in range(W)] for _ in range(H)]
      q = deque()

      start = (si, sj)
      if dist[si][sj] == -1:
        dist[si][sj] = 0
        q.append(start)

      while q:
        qi, qj = q.popleft()
        for di, dj in (-1, 0), (0, -1), (1, 0), (0, 1):
          ni = qi + di
          nj = qj + dj
          if ni < 0 or ni >= H:
            continue
          if nj < 0 or nj >= W:
            continue
          if S[ni][nj] == '#':
            continue
          if dist[ni][nj] == -1:
            dist[ni][nj] = dist[qi][qj]+1
            q.append((ni, nj))
            ans = max(ans, dist[ni][nj])
  return ans


if __name__ == "__main__":
  print(main())

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import queue
import math


def inputIntList():
  return [int(s) for s in input().split()]


def inputInt():
  return int(input())


def main():
  H, W = inputIntList()
  S = [[s for s in input()] for _ in range(H)]

  ans = 0
  for i in range(W):
    for j in range(H):
      dist = [[-1 for _ in range(W)] for _ in range(H)]
      if S[i][j] == '#':
        continue
      start = (i, j)
      q = queue.Queue()

      def update(pos, num):
        i, j = pos
        if dist[i][j] != -1:
          return
        dist[i][j] = num
        q.put(pos)
      update(start, 0)
      while not q.empty():
        qi, qj = q.get()
        for di, dj in zip([-1, 0, 1, 0], [0, -1, 0, 1]):
          ni = qi + di
          nj = qj + dj
          if ni < 0 or ni >= W:
            continue
          if nj < 0 or nj >= H:
            continue
          if S[ni][nj] == '#':
            continue
          update((ni, nj), dist[qi][qj]+1)
      ans = max(ans, max(max(x) for x in dist))
    return ans


if __name__ == "__main__":
  print(main())


Fixed submission:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from collections import deque
import math


def inputIntList():
  return [int(s) for s in input().split()]


def inputInt():
  return int(input())


def main():
  H, W = inputIntList()
  S = [[s for s in input()] for _ in range(H)]

  ans = 0
  for si in range(H):
    for sj in range(W):
      if S[si][sj] == '#':
        continue

      dist = [[-1 for _ in range(W)] for _ in range(H)]
      q = deque()

      start = (si, sj)
      if dist[si][sj] == -1:
        dist[si][sj] = 0
        q.append(start)

      while q:
        qi, qj = q.popleft()
        for di, dj in (-1, 0), (0, -1), (1, 0), (0, 1):
          ni = qi + di
          nj = qj + dj
          if ni < 0 or ni >= H:
            continue
          if nj < 0 or nj >= W:
            continue
          if S[ni][nj] == '#':
            continue
          if dist[ni][nj] == -1:
            dist[ni][nj] = dist[qi][qj]+1
            q.append((ni, nj))
            ans = max(ans, dist[ni][nj])
  return ans


if __name__ == "__main__":
  print(main())