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

my %pair_count;
for (<>) {
  $pair_count{$_}++ for get_pairs(grep length $_, split /\s/, $_);
}

my $comp = sub ($$) {-($pair_count{$_[0]} <=> $pair_count{$_[1]})};
print((sort $comp keys %pair_count)[0], $/);

sub get_range_iterator {
  my ($start, $end) = @_;
  return sub {
    if ($start <= $end) {
      return $start++;
    }
    else {
      return;
    }
  }
}

sub get_pairs {
  my $outer_iter = get_range_iterator(0, $#_);
  my $i = $outer_iter->();
  my @answer;
  while (defined($i)) {
    my $inner_iter = get_range_iterator($i+1, $#_);
    my $j = $inner_iter->();
    while (defined $j) {
      push @answer, join " ", sort @_[$i, $j];
      $j = $inner_iter->();
    }
    $i = $outer_iter->();
  }
  return @answer;
}