chomp (my @dta = <DATA>);
my ($ntot, $nmin) = split /\s/, shift @dta;
die if $nmin < 2 or $nmin > $ntot or $ntot != @dta;

sub cpfx { ($_[0]^$_[1]) =~ m/^\0*/s; substr($_[0], 0, length $&) }
sub sufs { map {[substr($_[0], -$_) => $_[1]]} 1..length($_[0]) }

my @SA = sort {$a->[0] cmp $b->[0]} map {sufs($dta[$_], $_)} 0..$#dta;
my @N = (0) x @dta;
my ($best, $n, $h, $t) = ("", -$nmin, 0, 0);

while ($h < @SA) {
  ($n += !$N[$SA[$h++]->[1]]++) >= 0 or next;
  ($n -= !--$N[$SA[$t++]->[1]]) while $n > 0 || $N[$SA[$t]->[1]] > 1;
  my $pfx = cpfx(map $SA[$_]->[0], $t, $h-1);
  $best = $pfx if length($best) < length($pfx);
}
print "\"$best\"\n";

__DATA__
6 3
You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue.  If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH, the string will grow to accommodate it.  To keep the string the
same length, you may need to pad or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is partly outside the string, only the part within the string is returned.  If the substring
is beyond either end of the string, substr() returns the undefined value and produces a warning.  When used as an lvalue, specifying a
substring that is entirely outside the string raises an exception.  Here's an example showing the behavior for boundary cases: