Given a string S, return the maximum number of letters a that can be inserted into S (including at the front and end of S) so that the resulting string doesn’t contain three consecutive letters a. If string S already contains the substring aaa, then your function should return -1.

A string aabaabaa can be created

A string aadaaoaagaa can be created

No longer string can be created because it already contains 'aaa'.

Given string contains a substring aaa, hence no additional 'a' can be inserted.