use strict;
use warnings;

my %pairs;

while (<DATA>) {
    chomp;
    next if ! length;

    my @pair = sort split;
    ++$pairs{$pair[0]}{$pair[1]};
}

my $totalHits = 0;
my %seconds;

for my $first (sort keys %pairs) {
    for my $second (sort keys %{$pairs{$first}}) {
        next if $pairs{$first}{$second} <= 1;
        ++$totalHits;

        my $previous =
            grep {exists $pairs{$_}{$first} && exists $pairs{$_}{$second}}
            keys %{$seconds{$first}};

        ++$seconds{$second}{$first};
        next if $previous;
        print "$first $second\n";
    }
}

print "$totalHits\n";

__DATA__
NP_01 NP_02
NP_02 NP_01
NP_01 NP_03
NP_03 NP_01
NP_02 NP_03
NP_03 NP_02
NP_04 NP_05