#!/usr/bin/env perl -l

use strict;
use warnings;

my @init = qw{1 20 2 0 2 1 3 0 3 0 3 1 7 0 8 2 9 1 9 2 10 3};
print "@init";

my %merge;
$merge{$init[$_]} += $init[$_+1] for grep { not $_ % 2 } 0 .. $#init; 

my @out = map { $_, $merge{$_} } sort { $a <=> $b } keys %merge;
print "@out";

##</code><code>##

1 20 2 0 2 1 3 0 3 0 3 1 7 0 8 2 9 1 9 2 10 3
1 20 2 1 3 1 7 0 8 2 9 3 10 3