using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Váltakozó_szöveg{
    class Program{
        static void Main(){
            string in1 = Console.ReadLine();
            string o = "";
            int[] a = new int[26];
            int n = in1.Length - 1;
            int m;
            foreach (char x in in1) a[(int)x-'a']++;
            if(2 * a.Max() > n + 1){Console.WriteLine("-1"); return;}
            for(char last='-'; n>=0; n--){
                m = a.Max();
                for(int i=0; i < 26; i++){
                    if (last != (char)(i + 'a') && a[i]>0 && (2 * m <= n || m == a[i])){
                        o += (last = (char)(i + 'a'));
                        a[i]--;
                        break;
                    }
                }
            }
            Console.WriteLine(o); ;
            Console.ReadKey();
        }
    }
}