function ROT13 (InputTxt as string) as string
    dim i as integer, ascVal as byte
    Result = ""

    for i = 1 to len(InputTxt)
        ascVal = asc(InputTxt[i])

        select case ascVal
        case 65 to 77, 97 to 109
            Result = Result + chr$(ascVal + 13)
        case 78 to 90, 110 to 122
            Result = Result + chr$(ascVal - 13)
        case else
            Result = Result + chr$(ascVal)
        end select
    next
end function

Input "Text to encode: "; a$
Print ROT13(a$)
Input "Press a key to end..."; a$