#include<iostream>

#include<cstring>

#include<string>

#include<cstdio>

#include<algorithm>

#include<vector>

#include<cmath>

#define maxn 1000050

using namespace std;

int sa[maxn + 50];

int lcp[maxn + 50];

int rk[maxn + 50];

int tmp[maxn + 50];

int d[maxn + 50][25];

int n, k;

bool cmp_sa(int i, int j)

{

	if (rk[i] != rk[j]) return rk[i] < rk[j];

	else{

		int ri = i + k <= n ? rk[i + k] : -1;

		int rj = j + k <= n ? rk[j + k] : -1;

		return ri < rj;

	}

}

void construct_sa(string S, int *sa)

{

	n = S.length();

	for (int i = 0; i <= n; i++){

		sa[i] = i;

		rk[i] = i < n ? S[i] : -1;

	}

	for (k = 1; k <= n; k <<= 1){

		sort(sa, sa + n + 1, cmp_sa);

		tmp[sa[0]] = 0;

		for (int i = 1; i <= n; i++){

			tmp[sa[i]] = tmp[sa[i - 1]] + (cmp_sa(sa[i - 1], sa[i]) ? 1 : 0);

		}

		for (int i = 0; i <= n; i++){

			rk[i] = tmp[i];

		}

	}

}

void construct_lcp(string S, int *sa, int *lcp)

{

	n = S.length();

	for (int i = 0; i <= n; i++) rk[sa[i]] = i;

	int h = 0;

	lcp[0] = 0;

	for (int i = 0; i < n; i++){

		int j = sa[rk[i] - 1];

		for (h ? h-- : 0; i + h < n&&j + h < n&&S[i + h] == S[j + h]; h++);

		lcp[rk[i] - 1] = h;

	}

}

void construct_rmq(int *lcp,int sizen)

{

	for (int i = 0; i <= sizen; i++) d[i][0] = lcp[i];

	for (int j = 1; (1 << j) <= sizen; j++){

		for (int i = 0; (i + (1 << j) - 1) <= sizen; i++){

			d[i][j] = min(d[i][j - 1], d[i + (1 << (j - 1))][j - 1]);

		}

	}

}

int rmq_query(int l, int r)

{

	if (l > r) swap(l, r); r -= 1;

	int k = 0; int len = r - l + 1;

	while ( (1 << (k + 1)) < len) k++;

	return min(d[l][k], d[r - (1 << k) + 1][k]);

}

string S;

string T;

int main()

{

	int ca = 0;

	while (cin >> S)

	{

		if (S == "END") break;

		int ctr = S.length();

		T = S;

		reverse(T.begin(), T.end());

		S += '#' + T;

		construct_sa(S, sa);

		construct_lcp(S, sa, lcp);

		construct_rmq(lcp, S.length() + 1);

		int ans = 0;

		string ansString;

		int ansid;

		int anstype;

		for (int i = 0; i < ctr; i++){

			int j = 2 * ctr - i;

			int l = rmq_query(rk[i], rk[j]);

			if (2 * l - 1>ans){

				ans = 2 * l - 1;

				ansid = i;

				anstype = 0;

			}

		}

		for (int i = 1; i < ctr; i++){

			int j = 2 * ctr - i + 1;

			int l = rmq_query(rk[i], rk[j]);

			if (2 * l > ans){

				ansid = i;

				anstype = 1;

				ans = 2 * l;

			}

		}

		if (anstype == 0){

			//ansString = S.substr(ansid-(ans-1)/2, ansid + (ans - 1) / 2);

			for (int i = ansid - (ans - 1) / 2; i <= ansid + (ans - 1) / 2; i++){

				cout << S[i];

			}

			cout << endl;

		}

		else{

			//ansString = S.substr(ansid - ans / 2, ansid + ans / 2);

			for (int i = ansid - ans/ 2; i <= ansid + ans/ 2-1; i++){

				cout << S[i];

			}

			cout << endl;

		}

	}

	return 0;

}