You are given a string, s, containing only uppercase English letters. You can perform operations on this string where, in each operation, you remove any occurrence of the substrings "AB" or "CD" from s.
Your task is to return the shortest string length after applying all possible operations.
Note: After each removal, the string joins back together, potentially creating new occurrences of "AB" or "CD" that can also be removed.
Constraints: