You are given a string s of length `n` containing only four kinds of characters: `'Q'`, `'W'`, `'E'`, and `'R'`.

A string is said to be balanced if each of its characters appears `n / 4` times where `n` is the length of the string.

Return the minimum length of the substring that can be replaced with any other string of the same length to make `s` balanced. If s is already balanced, return `0`.

Example 1:

    Input: s = "QWER"
    Output: 0
    Explanation: s is already balanced.

Example 2:

    Input: s = "QQWE"
    Output: 1
    Explanation: We need to replace a 'Q' to 'R', so that "RQWE" (or "QRWE") is balanced.

Example 3:

    Input: s = "QQQW"
    Output: 2
    Explanation: We can replace the first "QQ" to "ER".

Constraints:

- `n == s.length`
- `4 <= n <= 105`
- `n` is a multiple of `4`.
- `s` contains only `'Q'`, `'W'`, `'E'`, and `'R'`.