Given a binary string `s`, return the minimum number of character swaps to make it alternating, or `-1` if it is impossible.

The string is called alternating if no two adjacent characters are equal. For example, the strings `"010"` and `"1010"` are alternating, while the string `"0100"` is not.

Any two characters may be swapped, even if they are not adjacent.

Example 1:

    Input: s = "111000"
    Output: 1
    Explanation: Swap positions 1 and 4: "111000" -> "101010"
    The string is now alternating.

Example 2:

    Input: s = "010"
    Output: 0
    Explanation: The string is already alternating, no swaps are needed.

Example 3:

    Input: s = "1110"
    Output: -1

Constraints:

- `1 <= s.length <= 1000`
- `s[i]` is either `'0'` or `'1'`.