#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;
}
