0 1
1 2
1 3
1 4
2 4
4 5

##</code><code>##

#!/usr/bin/perl -w
use strict;

use Graph::Undirected;
my $net = Graph::Undirected->new;
while (<DATA>) {
    $net = $net->add_edge( split );
}

my @e = $net->edges();

print shift@e,shift@e,"\n" while @e;


__END__
0 1
1 0

##</code><code>##

01
01
10
10