FUNCTION rot13$(s$)
    LET ans$ = ""
    FOR a = 1 to len(s$)
        LET letter$ = (s$)[a:a+1-1]
        IF letter$ >= "A" and letter$ <= "Z" then
           LET char$ = chr$(ord(letter$[1:1])+13)
           IF char$ > "Z" then LET char$ = chr$(ord(char$[1:1])-26)
        ELSEIF letter$ >= "a" and letter$ <= "z" then
           LET char$ = chr$(ord(letter$[1:1])+13)
           IF char$ > "z" then LET char$ = chr$(ord(char$[1:1])-26)
        ELSE
           LET char$ = letter$
        END IF
        LET ans$ = ans$ & char$
    NEXT a
    LET s$ = ans$
    LET rot13$ = s$
END FUNCTION

INPUT prompt "Enter a string: ": s$
PRINT "Before encoding : "; s$
PRINT "After encoding  : "; rot13$(s$)
LET s$ = rot13$(s$)
PRINT "After decoding  : "; rot13$(s$)
END