H,W=map(int,input().split())
S=[input() for i in range(H)]
v=[[0]*W for i in range(H)]
for i in range(H):
  for j in range(W):
    if S[i][j]=='#':
      v[i][j]+=1
for i in range(H):
  for j in range(1,W):
    v[i][j]=v[i][j-1]+v[i][j]
for i in range(1,H):
  for j in range(W):
    v[i][j]=v[i-1][j]+v[i][j]
result=10**10
DX=[1,1,-1,-1]
DY=[1,-1,1,-1]
for i in range(H):
  for j in range(W):
    if S[i][j]=='.':
      continue
    for dx,dy in zip(DX,DY):
      x=i-dx
      y=j-dy
      t=0
      if x<0 or x>=H:
        t+=1
      else:
        if S[x][j]=='.':
          t+=1
      if y<0 or y>=W:
        t+=1
      else:
        if S[i][y]=='.':
          t+=1
      if t==2:
        l=1
        r=2000
        while True:
          m=(l+r+1)//2
          i2,j2=i+dx*(m-1),j+dy*(m-1)
          if i2<0 or i2>=H or j2<0 or j2>=W:
            r=m-1
          else:
            j3=max(j,j2)
            j4=min(j,j2)
            i3=max(i,i2)
            i4=min(i,i2)
            ans=v[i3][j3]
            if j4>0:
              ans-=v[i3][j4-1]
            if i4>0:
              ans-=v[i4-1][j3]
            if i4>0 and j4>0:
              ans+=v[i4-1][j4-1]
            if ans==m**2:
              l=m
            else:
              r=m-1
          if l==r:
            break
        result=min(result,l)
print(result)