using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            AGC022.A();
        }

        class AGC022
        {
            public static void A()
            {
                var S = Read.Str();
                if (S.Length < 26)
                {
                    for(int i = 0; i < 26; ++i)
                    {
                        if (!S.Contains((char)('a' + i)))
                        {
                            Console.WriteLine(S+ (char)('a' + i));
                            return;
                        }
                    }
                }
                else
                {
                    var last = -1;
                    for(int i = 0; i < 25; ++i)
                    {
                        if (S[i] < S[i + 1])
                        {
                            last = i;
                        }
                    }
                    if (last < 0)
                    {
                        Console.WriteLine(last);
                        return;
                    }
                    Console.WriteLine(S.Substring(0, last) + S.Skip(last + 1).Where(x => x > S[last]).Min());
                }
            }
            public static void B()
            {
                var N = Read.Int();
                HashSet<int> hash = new HashSet<int>();
                for (int i = 1; ; ++i)
                {
                    var mod6 = i % 6;
                    if (mod6 == 0 || mod6 == 2 || mod6 == 3 || mod6 == 4)
                    {
                        hash.Add(i);
                    }
                    if (hash.Count == N) { break; }
                }
                var sum = hash.Sum();
                switch (sum % 6)
                {
                    case 2:
                        hash.Remove(8); hash.Add(30000);
                        break;
                    case 3:
                        if (N == 3) { hash.Remove(4); hash.Add(25); }
                        else { hash.Remove(6); hash.Add(29997); }
                        break;
                    case 5:
                        hash.Remove(8); hash.Add(29997);
                        break;
                }
                Console.WriteLine(String.Join(" ", hash));
            }
        }

        class Util
        {

        }

        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new HashSet<int>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1]);
                    dict[ab[1]].Add(ab[0]);
                }
                return dict;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 13524