CF873B.Balanced Substring

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.

给你一个仅由字符 0 和 1 组成的字符串 $s$。$s$ 的一个子串 $[l,\,r]$ 定义为字符串 $s_l s_{l+1} s_{l+2} \dots s_r$，其长度为 $r - l + 1$。若一个子串中 0 的个数等于 1 的个数，则称该子串是平衡的。

你需要求出 $s$ 的最长平衡子串的长度。

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.

第一行包含一个整数 $n$（$1 \leq n \leq 100000$）——表示字符串 $s$ 的字符个数。

第二行包含一个长度恰好为 $n$ 的字符串 $s$。字符串 $s$ 中仅包含字符 0 和 1。

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

如果字符串 s 中不存在非空的平衡子串，则输出 0；否则，输出最长平衡子串的长度。

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.

在第一个例子中，你可以选择子串 [3, 6]。它是平衡的，且长度为 4。选择子串 [2, 5] 也是可行的。

在第二个例子中，无法找到一个非空的平衡子串。