get();
my @x = get().split(' ').map({.Int}).sort({ $^a <=> $^b });
my $diff = @x[1] - @x[0];
my $expect = @x[1];
my $ans  = True;

if $diff == 0 {
  $ans = False;
}
else {
  for 1..@x.end {
    if $expect != @x[$_] {
      $ans = False;
      last;
    }
    $expect += $diff;
  }
}

say($ans ?? 'YES' !! 'NO');