using System;
using System.Linq;
namespace Valtakozz
{

    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();

            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;
            for (int i = n; i > 0;)
            {
                mx = cnt.Max();
                for (int u = 0; u < 26; u++)
                {
                    current = (char)(u + 'a');
                    if (last != current && cnt[u] > 0 && (2 * mx <= n || mx == cnt[u]))
                    {
                        Console.Write(last = current);
                        cnt[u]--;
                        break;
                    }
                }
                i--;
            }

            Console.WriteLine();
        }
    }

}