#!/usr/bin/perl
use warnings;
use strict;

my $previous_num = -1;
my $previous_line;
my $print_next = 0;

sub output {
    my $num = shift;
    my $adjacent = abs($previous_num - $num) == 1;
    if ($print_next || $adjacent) {
        print $previous_line;
        $print_next = $adjacent;
    }
}

while (<>) {
    my ($num) = /([0-9]+$)/;
    output($num);
    $previous_num = $num;
    $previous_line = $_;
}
output($previous_num);  # Process the last line.