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

for 2..@x.end {
  if $diff == 0 {
    $ans = False;
    last;
  }
  $diff = @x[$_] - @x[$_-1];
}

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