#include <bits/stdc++.h>
using namespace std;
int main() {
    string s; cin >> s;
    if (s.size() == 1) {
        cout << s << '\n';
        return 0;
    }
    vector<int> a(26);
    for (int i = 0; i < s.size(); i++) {
        if (++a[s[i]-'a'] > (s.size()+1)/2) {
            cout << "-1";
            return 0;
        }
    }
    vector<int> out(s.size()+1);
    out[0] = -1;
    for (int i = 1; i <= s.size(); i++) {
        int x = 1000;
        for (int j = 0; j < 26; j++) {
            if (a[j] > 0 && out[i-1] != j && a[j] >= (s.size()-i+1)/2+1) {
                x = j;
                break;
            }
            if (a[j] > 0 && out[i-1] != j && x > j) x = j;
        }
        a[x]--;
        out[i] = x;
    }
    for (int i = 1; i <= s.size(); i++) cout << (char)(out[i]+'a');
}