#!/usr/bin/env perl

use strict;
use warnings;

$_ = <>;
my @v = sort {$a <=> $b} (split / /,<>);
my $count = $v[1] - $v[0];
my $prev = -1;
for my $x (@v) {
  if ($prev != -1 && ($prev == $x || $x - $prev != $count)) {
    print "NO\n";
    exit;
  }
  $prev = $x;
}
print "YES\n";