def r u;u==$p[u]?u:$p[u]=r($p[u]);end
n,m=gets.split.map &:to_i
a=(0..m).map{gets.split.map &:to_i}
$p=(0...m*n).to_a
print (0...m).any?{|x|(0...n).any?{|y|
  [[x+1,y],[x,y+1]].any?{|z,w|
    a[x][y]==a[z][w]&&(
      u=r x*n+y
      v=r z*n+w
      $p[u]=v
      u==v
    )
  }
}}?'':'im','possible'