manaker

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

const int MAXN = 500000;
std::array<char, MAXN> s;
std::array<int, MAXN> rad;

int main() {
    std::string str;
    std::cin >> str;
    int n = str.size();
    for (int i = 0; i < n; i++)
        s[i] = str[i];

    int ansL = 0;
    int ansR = 0;

    for (int z = 0; z < 2; ++z) {
        int l = 0, r = -1;
        for (int i = 0; i < n; ++i) {
            if (i > r) {
                rad[i] = 1 - z;
            }
            else {
                rad[i] = std::min(rad[l + r - i + z], r - i + z);
            }
            while (rad[i] + z <= i && i + rad[i] < n && s[i + rad[i]] == s[i - rad[i] - z]) {
                ++rad[i];
            }
            if (i + rad[i] - z > r) {
                l = i - rad[i] + (1 - z);
                r = i + rad[i] - 1;
            }
            if (ansR - ansL < i + rad[i] - (i - rad[i] - z) - 1) {
                ansL = i - rad[i] - z + 1;
                ansR = i + rad[i];
            }
        }
    }

    for (int i = ansL; i < ansR; i++)
        printf("%c", s[i]);
    return 0;

}