#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <math.h>
#include <stack>
#include <queue>
#include <iomanip>
#include <set>
#include <iterator>
#include <string>
#include <cstring>
#include <string.h>

#pragma comment(linker, "/STACK:1000000000")

#define INF 1000000000
#define MOD 1000000007
#define EPS 1e-8
#define pb push_back
//#define mp make_pair
#define f first
#define s second
#define all(xx) begin(xx), end(xx)
#define rep(i, l, r) for (int i = l; i < r; i++)
#define sz(v) (int)v.size()

using namespace std;

typedef long long ll;
typedef double dbl;
typedef long double ld;
typedef pair <int, int> pii;
typedef vector <int> vi;
typedef vector <pii> vii;
typedef vector <vi> vvi;
typedef vector <string> vs;
typedef map <int, int> mii;
typedef stack <int> si;

string str;
vi d1, d2;

int main() {
    //cout << fixed << setprecision(10);
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    //#ifdef LOCAL
//  freopen("input1.txt", "r", stdin);
//  freopen("output2.txt", "w", stdout);
    //#endif
    cin >> str;
    int n = str.size();
    d1.resize(n);
    d2.resize(n);
    int maxx = 0;
    int imx = 0, but = 0;
    int l = 0, r = -1;
    for (int i = 0; i < n; i++) {
        int k = (i > r ? 0 : min(d1[l + r - i], r - i + 1));
        while (i + k < n && i - k >= 0 && str[i + k] == str[i - k])
            k++;
        d1[i] = k--;
        if (maxx < d1[i]) {
            maxx = d1[i];
            imx = i;
        }
        if (i + k > r) {
            l = i - k;
            r = i + k;
        }
    }
    l = 0, r = -1;
    for (int i = 0; i < n; i++) {
        int k = (i > r ? 0 : min(d2[l + r - i + 1], r - i + 1));
        while (i + k < n && i - k > 0 && str[i - k - 1] == str[i + k]) {
            k++;
        }

        d2[i] = k;
        if ((maxx <= d2[i] && !but) || d2[i] > maxx) {
            maxx = d2[i];
            imx = i;
            but = 1;
        }
        if (i + k > r) {
            l = i - k - 1;
            r = i + k;
        }
    }
    for (int i = imx - maxx - but + 1; i < imx + maxx; i++) {
        cout << str[i];
    }
    return 0;
}