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

namespace Váltakozó{
    class Program{
        static void Main(){
            string str = Console.ReadLine();
            int N = str.Length;
            int[] C = new int[26];

            foreach (int i in str) C[i - 97]++;
            int max = C.Max(), last = -1;
            
            if(max > N / 2) {
                Console.WriteLine(-1); return;
            }

            string o1 = "";
            for(int i=N, j, v; i>0; i--){
                max = C.Max();
                for(j=0; j<26; j++){
                    v = C[j];
                    if(v>0 && last != j) if( max <= i/2 || max == v ){
                        C[j]--;
                        Console.Write((char)(j+97));
                        //o1 += (char)(j + 97);
                        last = j;
                        break;
                    }
                }
            }
            //Console.WriteLine(o1);
        }
    }
}