#!/usr/bin/perl
use strict;
my %h;
open F, "input.txt";
while (my $line=<F> ) {
    $line =~s/\s//;
    chomp $line;
    $h{$line}++;
}
close F;
while ((my $key, my $value)=each(%h)) {
    print $key."\n" if ($value==1);
    print $key."\n".$key."\n" if ($value==2);
}