#include<iostream>
#include<cmath>
#include<algorithm>
#include<iomanip>
#include<climits>
#include<numeric>
#include<set>
#include<sstream>
#include<map>
#include<vector>
#include<string>
#include<regex>

using namespace std;

const long long MOD = (long long)(1e9 + 7);

vector<string> split(const string& haystack, const string& needle) {
	vector<string> result;
	size_t start_pos = 0;
	size_t found_pos = haystack.find(needle, start_pos);
	while (found_pos != string::npos) {
		size_t count = found_pos - start_pos;
		string token = haystack.substr(start_pos, count);
		if (!token.empty()) {
			result.push_back(token);
		}
		start_pos = found_pos + needle.length();
		found_pos = haystack.find(needle, start_pos);
	}

	string last_token = haystack.substr(start_pos);
	if (!last_token.empty()) {
		result.push_back(last_token);
	}
	return result;
}

int main() {
	string s;cin >> s;
	int max_length = 0;
	map<int, int> prefix;
	int sum = 0;
	for (int i = 0;i < s.length();i++) {
		if (s[i] == '1') {
			sum++;
		}
		else {
			sum--;
		}

		if (prefix.find(sum) != prefix.end()) {
			max_length = max(max_length, i - prefix[sum]);
		}
		else {
			prefix[sum] = i;
		}

		if (sum == 0) {
			max_length = max(max_length, i + 1);
		}
	}
	cout << max_length;
	return 0;
}