$w, $h, = gets.split.map(&:to_i)
$list = $h.times.map { gets.split.map(&:to_i) }
$check_list = Array.new($h) {Array.new($w, false)}

def check_loop(x, y)
  if $check_list[y][x]
    return false
  end
  check_point(x, y)
end

def check_point(x, y, pre = nil)
  if $check_list[y][x]
    return true
  end
  $check_list[y][x] = true
  if pre != :r && x > 0 && $list[y][x] == $list[y][x - 1]
    if check_point(x - 1, y, :l)
      return true
    end
  end
  if pre != :d && y > 0 && $list[y][x] == $list[y - 1][x]
    if check_point(x, y - 1, :u)
      return true
    end
  end
  if pre != :l && x < $w - 1 && $list[y][x] == $list[y][x + 1]
    if check_point(x + 1, y, :r)
      return true
    end
  end
  if pre != :u && y < $h - 1 && $list[y][x] == $list[y + 1][x]
    if check_point(x, y + 1, :d)
      return true
    end
  end
  return false
end

looped = false
$h.times do |y|
  $w.times do |x|
    if check_loop(x, y)
      looped = true
      break
    end
    if looped
      break
    end
  end
end

require 'pp'
pp $list
pp $check_list

if looped
  puts 'possipble'
else
  puts 'impossible'
end