LOOP
  INPUT "": string$
  PRINT rot13$(string$)
ENDLOOP

FUNC rot13$(string$) CLOSED
  FOR pos#:=1 TO LEN(string$) DO
    x#:=ORD(string$(:pos#:))
    IF (x#>=97 AND x#<=109) OR (x#>=65 AND x#<=77) THEN
      x#:+13
    ELIF (x#>=110 AND x#<=122) OR (x#>=78 AND x#<=90) THEN
      x#:-13
    ENDIF
    string$(:pos#:):=CHR$(x#)
  ENDFOR pos#
  RETURN string$
ENDFUNC rot13$