<?php
$N = trim(fgets(STDIN));
$x = explode(" ", trim(fgets(STDIN)));
sort($x);
$d = $x[1] - $x[0];
$ans = "YES";
if($d > 0) {
  for($i = 0; $i < N - 1; $i++) {
    if(($x[$i + 1] - $x[$i]) != $d) $ans = "NO";
  }
} else {
  $ans = "NO";
}
print($ans);
?>