#include <iostream>
#include <string>
#include <cstring>

using namespace std;

void findBMstr(string str0){
    string str;
    str += "$#";
    int n=str0.size();
    for(int i = 0; i < n; i++){
        str += str0[i];
        str += "#";
    }
    int len=str.size();
    int *p = new int[len + 1];
    memset(p, 0, sizeof(p));
    int mx = 0, id = 0;
    for(int i = 1; i <=  len; i++){
        if(mx > i){
            p[i] = (p[2*id - i] < (mx - i) ? p[2*id - i] : (mx - i));
        }
        else{
            p[i] = 1;
        }
        while(str[i - p[i]] == str[i + p[i]])
            p[i]++;
        if(i + p[i] > mx){
            mx = i + p[i];
            id = i;
        }
    }
    int Max = 0, ii;
    for(int i = 1; i < len; i++){
        if(p[i] > Max){
            ii = i;
            Max = p[i];
        }
    }
    Max--;
    int s = ii - Max ;
    int e = ii + Max;
    for(int i = s; i <= e; i++){
        if(str[i] != '#'){
            cout << str[i];
        }
    }
    cout << endl;
    delete  p;
}
int main()
{
    string str;
    cin>>str;
    findBMstr(str);
    return 0;
}