uva11404

#include <iostream>
#include <istream>
#include <sstream>
#include <vector>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <numeric>
#include <chrono>
#include <ctime>
#include <cmath>
#include <cctype>
#include <string>
#include <cstdio>
#include <iomanip>


#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <iterator>
using namespace std;
const int maxn = 1007;
string dp[maxn][maxn],str;

string GetMax(string& a, string& b) {
	int alen = a.size(), blen = b.size();
	if (alen > blen) return a;
	else if (blen > alen) return b;
	return a > b ? b : a;
}

int main()
{
	while (cin >> str) {
		int n = str.size();
		string tmp;
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) dp[i][j] = "";
		}
		for (int i = n - 1; i >= 0; i--) {
			dp[i][i] = str[i];
			for (int j = i + 1; j < n; j++) {
				if (str[i] == str[j]) {
					tmp = str[i] + dp[i + 1][j - 1] + str[j];
					dp[i][j] = GetMax(dp[i][j], tmp);
				}
				else {
					dp[i][j] = GetMax(dp[i + 1][j], dp[i][j - 1]);
				}
			}
		}
		cout << dp[0][n - 1] << endl;
	}
}