#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <map>
using namespace std;
typedef long long ll;

string manachers_algorithm(string s) {
    if(s.empty()) {
        return "";
    }

    string t = "^";
    for(char c: s) {
        t += "#";
        t += c;
    }
    t += "#$";

    int n = (int) t.length();
    vector<int> P(n, 0);
    int C = 0, R = 0;
    int max_length = 0, center_idx = 0;

    for(int i = 1; i < n - 1; i++) {
        int i_mirror = 2 * C - i;

        if(R > i) {
            P[i] = min(R - i, P[i_mirror]);
        }

        while(t[i + 1 + P[i]] == t[i - 1 - P[i]]) {
            P[i]++;
        }

        if(i + P[i] > R) {
            C = i;
            R = i + P[i];
        }

        if(P[i] > max_length) {
            max_length = P[i];
            center_idx = i;
        }
    }
    int S = (center_idx - max_length) / 2;
    return s.substr(S, max_length);
}
int main() {
    ios_base::sync_with_stdio(false);
    string s;
    cin >> s;

    cout << manachers_algorithm(s) << endl;
    return 0;
}