Given a string containing one or more words, return an acronym of the words using the following constraints:
- The acronym should consist of the first letter of each word capitalized, unless otherwise noted.
- The acronym should ignore the first letter of these words unless they are the first word of the given string:
a, for, an, and, by, and of.
- The acronym letters should be returned in the order they are given.
- The acronym should not contain any spaces.