using System;
using System.Linq;
using System.Collections.Generic;
namespace Valtakozz
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine().Trim();
            string output = "";
            int[] cnt = new int[26];

            foreach (char c in s)
            {
                cnt[c - 'a']++;
            }

            int n = s.Length, mx = cnt.Max();
            char last = '-';

            if (2 * mx > n + 1)
            {
                Console.WriteLine("-1");
                return;
            }
            char current = '-';
            while (n >= 1)
            {
                mx = cnt.Max();
                for (int i = 0; i < 26; i++)
                {
                    if (last != (char)(i + 'a') && cnt[i] > 0 && (2 * mx <= n || mx == cnt[i]))
                    {
                        last = (char)(i + 'a');
                        output.Append((char)(i + 'a'));
                        cnt[i]--;
                        break;
                    }
                }
                n--;
            }
            Console.WriteLine(output);
        }
    }

}