You are given a string `s` and an integer `k`. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most `k` times.

Return the length of the longest substring containing the same letter you can get after performing the above operations.

Example 1:

    Input: s = "ABAB", k = 2
    Output: 4
    Explanation: Replace the two 'A's with two 'B's or vice versa.

Example 2:

    Input: s = "AABABBA", k = 1
    Output: 4
    Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA".
    The substring "BBBB" has the longest repeating letters, which is 4.

Constraints:

- `1 <= s.length <= 105`
- `s` consists of only uppercase English letters.
- `0 <= k <= s.length`