#include <bits/stdc++.h>

using namespace std;

#define int long long

signed main() {
    string s;
    cin >> s;
    int n = s.length();
    int nc = n;

    int chars[26] = {0};

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

    for(int i = 0; i < 26; i++) {
        if (chars[i] > n/2) {
            cout << -1 << endl;
            return 0;
        }
    }
    int jochar = -1, elozo = -2;

    for(int i = 0; i < n; i++) {
        for(int j = 25; j >= 0; j--) {
            if (chars[j] <= 0 || elozo == j) {
                continue;
            }
            if (nc%2 == 1 && chars[j] > nc/2) {
                jochar = j;
                j = -10;
            } else {
                jochar = j;
            }
        }
        
        
        cout << (char)(jochar+'a');
        //cout << nc << ' ';
        nc--;
        chars[jochar]--;
        elozo = jochar;
    }
	cout << endl;

    return 0;    
}