use strict;
use warnings;

my @list = sort {$a <=> $b} (1, 2, 3, 6, 9, 10, 13, 22, 20, 19, 15, 21);
printf "%s\n", join ",", @list;

my @newList;
my $runLen = 0;

for my $elt (@list, '') {
    if (! $runLen) {
        push @newList, $elt;
        $runLen = 1;
        next;
    }

    if ($elt eq '' || $runLen + $newList[-1] != $elt) {
        push @newList, "-", $runLen + $newList[-1] - 1 if $runLen != 1;
        push @newList, ',', $elt if $elt ne '';
        $runLen = 1;
    } else {
        ++$runLen
    }
}

print @newList, "\n";