sub get_nn_pairs {
    my @segment1 = @_;
    my @pairs;

    #  find and print all nn pairs form the sequence.

    foreach my $i (1 .. $#segment1) {
        push @pairs, "$segment1[$i-1]$segment1[$i]";
    }

    my %freq;
    foreach my $nn_pair (@pairs) {
        $freq{$nn_pair}++;
    }
    my @sorted_array = sort { $freq{$b} <=> $freq{$a} } keys %freq;
    return @sorted_array;
}


my @ordered_pairs = get_nn_pairs(@input);