INPUT "Enter a string: "; s$
ans$ = ""
FOR a = 1 TO LEN(s$)
        letter$ = MID$(s$, a, 1)
        IF letter$ >= "A" AND letter$ <= "Z" THEN
           char$ = CHR$(ASC(letter$) + 13)
           IF char$ > "Z" THEN char$ = CHR$(ASC(char$) - 26)
        else
           if letter$ >= "a" AND letter$ <= "z" THEN   char$ = CHR$(ASC(letter$) + 13)
           IF char$ > "z" THEN  char$ = CHR$(ASC(char$) - 26) ELSE  char$ = letter$
        END IF
        ans$ = ans$ + char$
NEXT a
PRINT ans$