h,w=gets.split.map &:to_i
a=[]
h.times do
  g=gets.chomp.split("")
  c=0
  w.times do |i|
    if g[i]==?.
      c=0
      g[i]=0
    else
      c+=1
      g[i]=c
    end
  end
  a<< g
end
w.times do |iw|
  c=0
  h.times do |ih|
    g=a[ih][iw]
    if g==0
      c=0
    else
      c+=1
      cc=c>g ? g:c
      t=true
      1.upto(cc-1) do |i|
        if a[ih-i][iw-i]<=(cc-i)
          cc=a[ih-i][iw-i]+1
          break
        end
      end
      a[ih][iw]=cc if g>cc
    end
  end
end
p (a.map{|i|i.max}.max/2.0).round