#include <iostream>
#include <vector>
#include <set>
#include <string>

using namespace std;

int main() {
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	string s = "";
	char c;
	set <char> sym;
	int i = 0;
	map <char, vector <int> > m;
	while (cin >> c) {
		s += c;
		sym.insert(c);
		m[c].push_back(i);
		++i;
	}
	string min = "";
	for (auto now : sym) {
		for (int j = 0; j < m[now].size(); ++j) {
			for (i = j + 1; i < m[now].size(); ++i) {
				string s1 = s.substr(m[now][j], (m[now][i] - m[now][j] + 1));
				string s2 = s1;
				reverse(s2.begin(), s2.end());
				if (s2 == s1 && (s1 < min || min == "")) {
					min = s2;
				}
			}
		}
	}
	cout << min << '\n';
	return 0;
}