#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']++;
    }

    int maxi = *max_element(betuk.begin(), betuk.end());
    if(maxi > n / 2 + n % 2){
        cout << "-1\n";
        return 0;
    }

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

    cout << "\n";
}