Final 2: The Chains

**Bài toán.** A necklace workshop throws bracket characters onto a bench in order. Find the length of the longest contiguous... no: longest **subsequence** (not necessarily contiguous) that forms a regular bracket sequence; print its length. **Constraints:** 1 ≤ |s| ≤ 200000; s contains only '(' and ')'. **Input:** one line: s. **Output:** one integer — the length.