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

using namespace std;

vector<string> split(string haystack, string needle) {
	vector<string> result;
	int start_pos = 0;
	int found_pos = haystack.find(needle, start_pos);
	while (found_pos != string::npos) {
		int 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 token = haystack.substr(start_pos, haystack.length()-start_pos);
	if (!token.empty()) {
		result.push_back(token);
	}
	return result;
}
string nameConvert(const string& name) {
	const string DELIMITER = " ";
	stringstream builder;
	auto parts = split(name, DELIMITER);
	for (auto part : parts) {
		part[0] = toupper(part[0]);
		transform(part.begin()+1, part.end(),part.begin()+1, ::tolower);
		builder << part << DELIMITER;
	}
	return builder.str();
}
string dateConvert(const string& date) {
	const string DELIMITER = "/";
	stringstream builder;
	auto parts = split(date, DELIMITER);
	builder << setw(2) << setfill('0') << parts[0] << DELIMITER;
	builder << setw(2) << setfill('0') << parts[1] << DELIMITER;
	builder << parts[2];
	return builder.str();

}

int main() {
	string s;cin >> s;
	string maxResult = "";
	maxResult.push_back(s[0]);

	string currentResult = "";
	currentResult.push_back(s[0]);

	for (int i = 1;i < s.size();i++) {
		if (s[i] != s[i - 1]) {
			currentResult = currentResult + s[i];
		}
		else {
			if (currentResult.size() > maxResult.size()) {
				maxResult = currentResult;
			}
			else if (currentResult.size() == maxResult.size()) {
				maxResult = max(maxResult, currentResult);
			}
			currentResult = "";
			currentResult.push_back(s[i]);
		}
	}
	if (currentResult.size() > maxResult.size()) {
		maxResult = currentResult;

	}
	else if (currentResult.size() == maxResult.size()) {
		maxResult = max(maxResult, currentResult);
	}
	cout << maxResult;

}