#!/usr/bin/env perl

use strict;
use warnings;

$_ = <>;
my @v = split / /,<>;
my %h;
my $count = abs($v[0] - $v[1]);
my $prev = -1;
for my $x (@v) {
  $h{$x} = 1;
  if ($prev != -1 && abs($prev - $x) != $count) {
    print "NO\n";
    exit;
  }
  $prev = $x;
}
my $size = int(keys %h);
if ($size != @v) {
  print "NO\n";
  exit;
}
print "YES\n";