with Ada.Text_IO; use Ada.Text_IO;
with Ada.Characters.Handling; use Ada.Characters.Handling;

procedure rot13 is
  rot13_table : constant array (Positive range <>) of Character := ('N','O',
                        'P','Q','R','S','T','U','V','W','X','Y','Z','A',
                        'B','C','D','E','F','G','H','I','J','K','L','M');
  zeichen : Character;
  c : Integer;
begin
  loop
    Get_Immediate(zeichen);
    c := Character'Pos(zeichen);
    case c is
      when 65 .. 90 => zeichen := rot13_table(c-64);
      when 97 .. 122 => zeichen := To_Lower(rot13_table(c-96));
      when others => null;
    end case;
    Put(zeichen);
  end loop;

  exception
    when END_ERROR => null;
end rot13;