//#pragma comment(linker, "/STACK:16777216") //for c++ Compiler

#include <stdio.h>

#include <iostream>

#include <string>

#include <algorithm>

using namespace std;

string a, res, tmp1, tmp2;

int ans;

int main(){

    int i, j, k, t, n, m;

    while(cin >> a){

        ans = 1;

        int len = a.size();

        bool flag = false;

        for(i = 0; i < len; ++i){

            if(len - i < ans)   break;

            for(j = len - i; j >= 1; --j){

                tmp1 = a.substr(i, j);

                tmp2 = tmp1;

                reverse(tmp1.begin(), tmp1.end());a

                if(!flag){

                    if(tmp1.compare(tmp2) == 0 && j >= ans){

                        flag = true;

                        res = tmp1;

                        ans = j;

                        break;

                    }

                } else{

                    if(tmp1.compare(tmp2) == 0 && j > ans){

                        flag = true;

                        res = tmp1;

                        ans = j;

                        break;

                    }

                }

            }

        }

        cout << res << endl;

    }

    return 0;

}