//#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;
}