#include <bits/stdc++.h>

using namespace std;

typedef pair<int, int> pii;

int main()
{
    string s;
    cin >> s;

    int n = s.size();

    vector<int> betuk(26, 0);
    vector<pii> sorrend;

    for(int i = 0; i < n; i++){
        betuk[s[i] - 'a']++;
    }

    for(int i = 0; i < 26; i++){
        if(betuk[i] > n / 2 + (n % 2)){
            cout << "-1\n";
            return 0;
        }
    }

    int maxi = *max_element(betuk.begin(), betuk.end());
    int utolso = -1;
    for(int i = 0; i < n; i++){
        maxi = *max_element(betuk.begin(), betuk.end());
        for(int i = 0; i < 26; i++){
            if(utolso != i && betuk[i]){
                if(2 * maxi >= n || maxi == betuk[i]){
                    cout << (char)(i + 'a');
                    utolso = i;
                    betuk[i]--;
                    break;
                }
            }
        }
    }

    cout << "\n";
}