Balanced Substring

给出一个长度为 $n$ 的 $01$ 串，其中含有相同 $0,1$ 个数的子串被称为“平衡串”，问最长的平衡串的长度。

You are given a string $ s $ consisting only of characters 0 and 1. A substring $ [l,r] $ of $ s $ is a string $ s_{l}s_{l+1}s_{l+2}...\ s_{r} $ , and its length equals to $ r-l+1 $ . A substring is called balanced if the number of zeroes (0) equals to the number of ones in this substring. You have to determine the length of the longest balanced substring of $ s $ .

The first line contains $ n $ ( $ 1<=n<=100000 $ ) — the number of characters in $ s $ . The second line contains a string $ s $ consisting of exactly $ n $ characters. Only characters 0 and 1 can appear in $ s $ .

If there is no non-empty balanced substring in $ s $ , print 0. Otherwise, print the length of the longest balanced substring.

8
11010111

4

3
111

0

In the first example you can choose the substring $ [3,6] $ . It is balanced, and its length is $ 4 $ . Choosing the substring $ [2,5] $ is also possible. In the second example it's impossible to find a non-empty balanced substring.