#include <bits/stdc++.h>
using namespace std;
int main()
{
	string s;
	getline(cin,s);
	int len = s.size(); 
	for (int i = 1;i <= len;i++){
		for (int j = 0;j <= i + j - 1;j++){
			string s1 = s.substr(j,i);
			string s2 = s1;
			reverse(s.begin(),s.end());
			if (s1 == s2){
				cout << s1;
			}
		}
	}
	return 0;
}