#include <iostream>
#include <algorithm>
#include <functional>
#include <sstream>
using namespace std;

void processString(string &input){
	string result;
	remove_copy_if(input.begin(), input.end(), back_inserter(result), ptr_fun<int, int>(&ispunct));
	for_each(result.begin(), result.end(), [](char &c){
		c = ::toupper(c);
	});
	input = result;
}

void solve(string input){
	processString(input);
    int maxLength = 1;
    int low, high, start = 0;
    int len = input.length();
    for(int i = 1; i < len; ++i){
        low = i - 1;
        high = i + 1;
        while(input[low] == input[high] && low >= 0 && high < len){
            if(high - low + 1 > maxLength){
                start = low;
                maxLength = high - low + 1;
            }
            --low;
            ++high;
        }
        low = i - 1;
        high = i;
        while(input[low] == input[high] && low >= 0 && high < len){
            if(high - low + 1 > maxLength){
                start = low;
                maxLength = high - low + 1;
            }
            --low;
            ++high;
        }
    }
    cout << input.substr(start, maxLength) << endl;
}

int main(){
	string in;
	while(cin >> in){
		solve(in);
	}
	return 0;
}