let chan = ref stdin

let rot13 c =
  let x = int_of_char c in
    if x >= 97 && x < 110 || x >= 65 && x < 78 then
      char_of_int (x + 13)
    else if x >= 110 && x < 123 || x >= 78 && x < 91 then
      char_of_int (x - 13)
    else c
;;

let main argc argv =
  if argc = 2 then
    begin
      try
	chan := open_in argv.(1)
      with Sys_error s -> print_endline s; exit 0
    end;
  while true do
    try
      let c = input_char !chan in
	print_char (rot13 c); flush stdout
    with End_of_file -> exit 0
  done
;;

main (Array.length Sys.argv) Sys.argv