// Miroslaw
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cctype>
#include<cmath>
#include<cassert>
#include<iostream>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
using namespace std;

typedef long long LL;
typedef vector<int> VI;

#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORD(i,a,b) for(int i=(a);i>=(b);i--)
#define FOREACH(i,c) for(__typeof((c).begin())i = (c).begin();i!=(c).end(); ++i)

int cond = 1;
#define DB(X) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<" = "<<X<<endl;}}
#define DB2(X, Y) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<" = "<<X<<", "<<#Y<<" = "<<Y<<endl;}}
#define DB3(X, Y, W) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<" = "<<X<<", "<<#Y<<" = "<<Y<<", "<<#W<<" = "<<W<<endl;}}
#define DB4(X, Y, W, Z) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<" = "<<X<<", "<<#Y<<" = "<<Y<<", "<<#W<<" = "<<W<<", "<<#Z<<" = "<<Z<<endl;}}
#define DBP(X) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<" = ("<<X.first<<", "<<X.second<<")" << endl;}}
#define DBV(X) {if(cond){cerr<<"Line:"<<__LINE__<<", "<<#X<<": ";FOREACH(__i,X)cerr<<*__i<<" ";cerr<<endl;}}

// Std. implementation of Manacher algorithm
vector<int> manacher(const string& s, int parity) {
  vector<int> R(s.size(), 0);
  int i = 1, j = 0, c = 1;
  while (i <= s.size() - 1) {
    while (s[i - j - parity] == s[i + j + 1]) {
      j++;
    }
    R[c++] = j;
    int k = 1;
    while (R[i - k] != R[i] - k && k <= j) {
      R[c++] = min(R[i - k], R[i] - k);
      k++;
    }
    j=max(j - k, 0);
    i += k;
  }
  return R;
}

int main() {
  string s;
  cin >> s;
	int ms = -1, msloc = 0;
  vector<int> v = manacher("#" + s + "@", 0);
  for (int i = 1; i < v.size() - 1; ++i) {
    if (v[i] * 2 > ms) {
      ms = 2 * v[i];
      msloc = i;
    }
  }
//  DBV(v);
  vector<int> v2 = manacher("#" + s + "@", 1);
  for (int i = 1; i < v2.size() - 1; ++i) {
    if (v2[i] * 2 + 1 > ms) {
      ms = 2 * v2[i] + 1;
      msloc = i;
    }
  }
//  DBV(v2);
//  DB2(ms, msloc);
  for (int i = msloc - ms / 2 - ms %2; i < msloc + ms / 2; ++i) {
    printf("%c", s[i]);
  }
  printf("\n");
  return 0;
}