You are given a 0-indexed string `s` of even length `n`. The string consists of exactly `n / 2` opening brackets `'['` and `n / 2` closing brackets `']'`.

A string is called balanced if and only if:

- It is the empty string, or
- It can be written as `AB`, where both `A` and `B` are balanced strings, or
- It can be written as `[C]`, where `C` is a balanced string.

You may swap the brackets at any two indices any number of times.

Return the minimum number of swaps to make `s` balanced.

Example 1:

    Input: s = "][]["
    Output: 1
    Explanation: You can make the string balanced by swapping index 0 with index 3.
    The resulting string is "[[]]".

Example 2:

    Input: s = "]]][[["
    Output: 2
    Explanation: You can do the following to make the string balanced:
    - Swap index 0 with index 4. s = "[]][][".
    - Swap index 1 with index 5. s = "[[][]]".
    The resulting string is "[[][]]".

Example 3:

    Input: s = "[]"
    Output: 0
    Explanation: The string is already balanced.

Constraints:

- `n == s.length`
- `2 <= n <= 106`
- `n` is even.
- `s[i]` is either `'['` or `']'`.
- The number of opening brackets `'['` equals `n / 2`, and the number of closing brackets `']'` equals `n / 2`.