#!/usr/local/bin/perl

sub make_tr {
  my($search, $replace, $flags) = @_;

  $flags = '' if not defined $flags;

  my $code = "sub { \$_[0] =~ tr/$search/$replace/$flags }";
  my $sub = eval $code;
  die "$code\n$@" if $@;
  return $sub;
}

my $rotate = shift;
$rotate = 13 unless defined $rotate and $rotate =~ /^\d+\z/;

$rotate %= 26;

$replace = chr(ord('A')+$rotate) . '-ZA-' . chr(ord('A')+$rotate-1);
$replace .= lc($replace);

my $rotate_sub = make_tr('A-Za-z', $replace, '');

while (<>) {
  $rotate_sub->($_);
  print;
}