import sys
input=lambda: sys.stdin.readline().rstrip()
w,h=map(int,input().split())
C=[input() for _ in range(h)]
n=w*h
edge=[[] for i in range(n)]

import heapq
def dijkstra_heap(s):
  d=[float("inf")]*n
  used=[True]*n
  d[s]=0
  used[s]=False
  edgelist=[]
  for e in edge[s]:
    heapq.heappush(edgelist,e)
  while edgelist:
    minedge=heapq.heappop(edgelist)
    minedge_cost=minedge//(10**6)
    minedge_v=minedge%(10**6)
    if not used[minedge_v]:
      continue
    d[minedge_v] = minedge_cost
    used[minedge_v] = False
    for e in edge[minedge_v]:
      e_cost=e//(10**6)
      e_v=e%(10**6)
      if used[e_v]:
        heapq.heappush(edgelist,e+minedge_cost*10**6)
  return d
S=[]
for i in range(h):
  for j in range(w):
    if C[i][j]=="." and not S:
      S.append(i*w+j)
    if i!=0:
      if C[i-1][j]=="#":
        edge[i*w+j].append(10**6+(i-1)*w+j)
      else:
        edge[i*w+j].append((i-1)*w+j)
    if j!=w-1:
      if C[i][j+1]=="#":
        edge[i*w+j].append(10**6+i*w+j+1)
      else:
        edge[i*w+j].append(i*w+j+1)
    if i!=h-1:
      if C[i+1][j]=="#":
        edge[i*w+j].append(10**6+(i+1)*w+j)
      else:
        edge[i*w+j].append((i+1)*w+j)
    if j!=0:
      if C[i][j-1]=="#":
        edge[i*w+j].append(10**6+i*w+j-1)
      else:
        edge[i*w+j].append(i*w+j-1)

D=dijkstra_heap(S[0])
ans=0
for i in range(h):
  for j in range(w):
    if C[i][j]==".":
      ans=max(ans,D[i*w+j])
print(ans)