Compute unmatched parentheses length

## Problem You are given a string `s` consisting only of characters `'('` and `')'`. Repeatedly remove any **matched pair** of parentheses as part of a valid parentheses matching (equivalently, cancel out all valid matches). After all possible matches are removed, return the **number of remaining characters** (the count of unmatched parentheses). ### Examples - `s = "()"` → all matched → return `0` - `s = "())"` → one pair matched, one `')'` unmatched → return `1` - `s = "(()"` → one pair matched, one `'('` unmatched → return `1` - `s = ")()("` → two matched in the middle, ends unmatched → return `2` ### Function signature - Input: `s: string` - Output: `int` ### Constraints - `0 <= len(s) <= 10^6` (design for linear time) ### Notes - You only need to return the number of unmatched parentheses, not the corrected string.