def g;gets.split.map(&:to_i);end
w,h=g
$m=(1..h).map{g+[0]}+[0]*w

$checked={}

def dfs(x,y)
  return if $checked[[x,y]]
  r=[[0,-1],[1,0],[-1,0],[0,1]]
  c=$m[y][x]
  stack=[[-1,x,y]]
  while stack[0]
    dir,tx,ty=stack.pop
    if $m[ty][tx]==c
      return true if $checked[[tx,ty]]
      $checked[[tx,ty]]=true
      4.times{|i|i!=dir&&stack.push([3-i,tx+r[i][0],ty+r[i][1]])}
    end
  end
end

puts (0...h).any?{|y|(0...w).any?{|x|dfs(x,y)}}?"possible":"impossible"