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

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

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

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

  rcdist=0

  do i=0,H+1
     do j=0,W+1
        if (map(i,j).eq.umi) then
           if(map(i+1,j+1).eq.riku) rcdist(i+1,j+1)=1
           if(map(i+1,j-1).eq.riku) rcdist(i+1,j-1)=1
           if(map(i+1,j  ).eq.riku) rcdist(i+1,j  )=1
           if(map(i-1,j+1).eq.riku) rcdist(i-1,j+1)=1
           if(map(i-1,j-1).eq.riku) rcdist(i-1,j-1)=1
           if(map(i-1,j  ).eq.riku) rcdist(i-1,j  )=1
           if(map(i  ,j+1).eq.riku) rcdist(i  ,j+1)=1
           if(map(i  ,j-1).eq.riku) rcdist(i  ,j-1)=1
        end if
     end do
  end do

  ! propagation
  do l=2,MAX(H,W)
     do i=1,W
        do j=1,H
           if (map(i,j).eq.riku .and. rcdist(i,j).eq.l-1) then
              if(map(i+1,j+1).eq.riku .and. rcdist(i+1,j+1).eq.0) rcdist(i+1,j+1)=l
              if(map(i+1,j-1).eq.riku .and. rcdist(i+1,j-1).eq.0) rcdist(i+1,j-1)=l
              if(map(i+1,j  ).eq.riku .and. rcdist(i+1,j  ).eq.0) rcdist(i+1,j  )=l
              if(map(i-1,j+1).eq.riku .and. rcdist(i-1,j+1).eq.0) rcdist(i-1,j+1)=l
              if(map(i-1,j-1).eq.riku .and. rcdist(i-1,j-1).eq.0) rcdist(i-1,j-1)=l
              if(map(i-1,j  ).eq.riku .and. rcdist(i-1,j  ).eq.0) rcdist(i-1,j  )=l
              if(map(i  ,j+1).eq.riku .and. rcdist(i  ,j+1).eq.0) rcdist(i  ,j+1)=l
              if(map(i  ,j-1).eq.riku .and. rcdist(i  ,j-1).eq.0) rcdist(i  ,j-1)=l
           endif
        end do
     end do

     min_cdist=1
     max_cdist=0
     do i=1,W
        do j=1,H
           if(map(i,j).eq.riku) then
              min_cdist = MIN(rcdist(i,j),min_cdist)
              max_cdist = MAX(rcdist(i,j),max_cdist)
           end if
        end do
     end do
     if (min_cdist .eq. 0) then
        print '(i0)',max_cdist
        exit
     end if
  end do
end program