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

my $file = $ARGV[0] or die "Usage: $0 <input_file>";
open(my $fh, '<', $file) or die "Unable to open '$file' for reading: $!";

my %pair;
while (<$fh>) {
    tr/\r\n//d;
    my @word = sort split ' ';
    my $next = combo(2, @word);
    while (my @combo = $next->()) {
        my $key = join "\0", @combo;
        ++$pair{$key};       
    }
}

my $comp = sub { return $pair{$_[0]} > $pair{$_[1]} ? $_[0] : undef; };
my $maxval = gen_reduce(compare => $comp);

$maxval->($_) for keys %pair;
print $maxval->(), "\n";

# taken from [id://393064]
sub combo {
    my $by = shift;
    return sub { () } if ! $by || $by =~ /\D/ || @_ < $by;
    my @list = @_;

    my @position = (0 .. $by - 2, $by - 2);
    my @stop     = @list - $by .. $#list;
    my $end_pos  = $#position;
    my $done     = undef;

    return sub {
        return () if $done;
        my $cur = $end_pos;
        {
            if ( ++$position[ $cur ] > $stop[ $cur ] ) {
                $position[ --$cur ]++;
                redo if $position[ $cur ] > $stop[ $cur ];
                my $new_pos = $position[ $cur ];
                @position[ $cur .. $end_pos ] = $new_pos .. $new_pos + $by;
            }
        }
        $done = 1 if $position[0] == $stop[0];
        return @list[ @position ];
    }
}

# Taken from [id://492651]
sub gen_reduce {
    my $usage = 'Usage: gen_reduce("initial" => $val, "compare" => $code_ref)';

    die $usage if @_ % 2;
    my %opt = @_;

    die $usage if ! defined $opt{compare} || ref $opt{compare} ne 'CODE';
    my $compare = $opt{compare};
    my $val = $opt{initial};

    return sub {
        for ( @_ ) {
            $val = $_ if ! defined $val || $compare->($_,  $val);
        }
        return $val;
    };
}