#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

std::string manacher(const std::string &str)
{
    int n = 2 * str.size() + 1;

    std::vector<int> d(n);

    int c = 0, r = 0;
    for (int i = 0; i < n; i++) {
        int i_prime = c - (i - c);
        if (i < r && i + d[i_prime] < r) {
            d[i] = d[i_prime];
        } else {
            d[i] = i <= r ? r - i : 0;

            if ((i + d[i]) % 2 == 1) {
                d[i]++;
            }

            while (i - d[i] - 1 >= 0 && i + d[i] + 1 < n &&
                   str[(i - d[i] - 1) / 2] == str[(i + d[i] + 1) / 2]) {
                d[i] += 2;
            }

            if (i + d[i] > r) {
                c = i;
                r = i + d[i];
            }
        }
    }

    int max_i = 0;
    for (int i = 0; i < n; i++) {
        if (d[max_i] < d[i]) {
            max_i = i;
        }
    }

    return str.substr((max_i - d[max_i]) / 2, d[max_i]);
}

int main(void)
{
    std::string str;
    std::cin >> str;

    std::cout << manacher(str) << std::endl; 

    return 0;
}