<?php
  $fileout = fopen("output.txt","w");
  $arra = file("input.txt");
  $n = array_shift($arra);
  for($i = 0; $i < $n; $i++){
    if (preg_match("/".$arra[$i][0]."{".(int)$n."}/", $arra[$i])){
      $answer[] = $arra[$i][0];
    }
  }
  if (!count($answer)){
    for($i = 0; $i < $n; $i++){
      $cur='';
      for($j = 0; $j < $n; $j++){
        $cur=$cur.$arra[$j][$i];
      }
      if (preg_match("/".$cur[0]."{".(int)$n."}/", $cur)){
        $answer[] = $cur[0];
      }
    }
  }
  if (!count($answer)){
    fwrite($fileout, 'NO');
  } else {
    sort($answer, SORT_STRING);
    fwrite($fileout, implode("", array_unique($answer)));
  }
?>