integer(kind=2)::H,W
  character,allocatable::map(:,:)
  character,parameter::riku='#',umi='.'
  logical,allocatable::covered(:,:)
  character*3000::string
  integer::cdist

  read *,H,W
  allocate(map(0:W+1,0:H+1))
  allocate(covered(0:W+1,0:H+1))

  do i=1,H
     read *, string
     do j=1,W
        map(j,i)=string(j:j)
     end do
  end do

  map(0  ,:)  =umi
  map(W+1,:)  =umi
  map(:  ,0)  =umi
  map(:,H+1)  =umi

  do i=0,W+1
     do j=0,H+1
        if (map(i,j).eq.umi) then
           covered(i,j)=.true.
        else
           covered(i,j)=.false.
        end if
     end do
  end do

!  check cover all riku in cdist
  do cdist=1, MAX(H,W)
     do i=0,W+1
        do j=0,H+1
           if (map(i,j).eq.umi) then
              covered(MIN(i+cdist,W+1),MAX(0,j-cdist):MIN(j+cdist,H+1))=.true.
              covered(MAX(i-cdist,0),  MAX(0,j-cdist):MIN(j+cdist,H+1))=.true.
              covered(MAX(0,i-cdist):MIN(i+cdist,W+1),MIN(j+cdist,H+1))=.true.
              covered(MAX(0,i-cdist):MIN(i+cdist,W+1),MAX(j-cdist,0))  =.true.
           endif
        end do
     end do
     if ( ALL(covered).eqv..true.) then
        print '(i0)',cdist
        exit
     end if
  end do
end program