Given two integers `a` and `b`, return any string `s` such that:

- `s` has length `a + b` and contains exactly `a` `'a'` letters, and exactly `b` `'b'` letters,
- The substring `'aaa'` does not occur in `s`, and
- The substring `'bbb'` does not occur in `s`.

Example 1:

    Input: a = 1, b = 2
    Output: "abb"
    Explanation: "abb", "bab" and "bba" are all correct answers.

Example 2:

    Input: a = 4, b = 1
    Output: "aabaa"

Constraints:

- `0 <= a, b <= 100`
- It is guaranteed such an `s` exists for the given `a` and `b`.