sub f {
@a = split //, shift; $ih = shift;
for $i (0 .. $#a){
        $e = $a[$i]; ${$ih}{$e} = 1;
        for $y ($i+1 .. $#a){
                $e .= $a[$y]; ${$ih}{$e} = 1;
        }
}
}

($l, $m) = split /\s/, <DATA>;
$_ = <DATA>; chomp; %h = (); f($_, \%h);

while(<DATA>){
        chomp; %th = (); f($_, \%th);
        $h{$_}++ foreach (keys %th);
}

foreach $key (keys %h){
        if($h{$key} == $m){
                $r[length($key)] = [] if ! exists $r[length($key)];
                push $r[length($key)], $key;
        }
}

print "@{$r[$#r]}\n";

__DATA__
3 2
ac
bc
b