Problem: The ROT13 Excel function allows you to scramble a message by applying a simple rule: every character is replaced by the character 13 characters away from it in the alphabet, with wrap-around applied if the letter is closer than 13 from the end of the alphabet. In other words, letter 'A' (1) is replaced with 'N' (14), and 'B' (2) is replaced with 'O' (15), while 'P' (16) is replaced with 'C' (3), and so on. custom message scrambling UDF in Excel which allows you to specify a different offset than just 13, but provides the same results otherwise. Presumably, you'd pass it a string and a number, and expect a scrambled string back. Don't change any character which isn't either a lower-case or upper-case letter, and also check that the offset provided is a valid argument. Demonstrate that your UDF works by using vba it on a spreadsheet. Here are some sample values to test your work.