#include <bits/stdc++.h>
#define int unsigned long long
using namespace std;
const int N = 1000000 + 10;
const int P = 131;
string s;
int Pre[N],ans;
int lft,rgt;
int Base[N];
int calc(int lft,int rgt) {
	int length = rgt - lft + 1;
	return Pre[rgt] - Pre[lft - 1] * Base[length];
}
bool check(int len) {
	if(calc(1,len) != calc(s.size() - len + 1,s.size())) return false;
	int res = calc(1,len);
	for(int i = 2; i + len - 1 < s.size(); i++) {
		int rgt = i + len - 1;
		if(calc(i,rgt) == res) return true;
	}
	return false;
}
signed main() {
	cin >> s;
	Base[0] = 1;
	for(int i = 1; i <= s.size() + 1; i++) {
		Base[i] = Base[i - 1] * P;
	}
	for(int i = 1; i <= s.size(); i++) {
		Pre[i] = Pre[i - 1] * P + s[i - 1] - 'a' + 1;
	}
	int flag = false;
	for(ans = s.size() - 2; ans >= 1; ans--) {
		if(check(ans)){
		    for(int i = 0; i < ans; i++) cout << s[i];
	        return 0;
		}
	}
	puts("-1");
	return 0;
}

//3 abcbac