Given a string s consisting only of the characters '[' and ']', you may perform swaps of any two characters in s. Return the minimum number of swaps required to transform s into a balanced bracket string.

A string is balanced if it can be read from left to right such that, at every position, the number of '[' seen so far is at least the number of ']' seen so far, and the total counts of '[' and ']' are equal.

Constraints:

• n == s.length

• $2 \leq$ n $\leq 10^6$

• n is even

• s[i] is either '[' or ']'

• The number of '[' in s equals n / 2, and the number of ']' in s equals n / 2

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