def f(board, width, height)
  visited = height.times.map{[0]*width}
  g = lambda{|h, w, n, cnt=0, from=nil|
    return if !h.between?(0, height-1)
    return if !w.between?(0, width-1)
    return if n != board[h][w]
    if visited[h][w] > 0
      # p [n, cnt, [h,w]]
      return cnt + visited[h][w] > 4
    end
    cnt += 1
    visited[h][w] = cnt
    from != :l && g.(h, w-1, n, cnt, :r) ||
    from != :r && g.(h, w+1, n, cnt, :l) ||
    from != :u && g.(h-1, w, n, cnt, :d) ||
    from != :d && g.(h+1, w, n, cnt, :u)
  }
  # visited.each{|l| puts l.join}

  height.times{|h|
    width.times{|w|
      next if visited[h][w] > 0
      if g.(h, w, board[h][w])
        # p [h,w]
        # visited.each{|l| puts l.join}
        return true
      end
    }
  }
  # visited.each{|l| puts l.join}
  false
end

W, H = gets.split.take(2).map(&:to_i)
M = H.times.map{gets.split.take(W).map(&:to_i)}
puts f(M, W, H) ? :possible : :impossible