using System;

namespace Valtakozz
{
    using System;
    using System.Linq;

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

            int[] cnt = new int[26];
            Array.Fill(cnt, 0);

            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;
            }

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

                n--;
            }

            Console.WriteLine();
        }
    }

}