using System;

public class Program
{
    private char[] A;
    private int N;

    public void Solve()
    {
        var sc = new Scanner();
        A = sc.Next().ToCharArray();
        N = A.Length;

        var nextIndex = new int[N + 2, 26];
        var tmpIndex = new int[256];
        for (char c = 'a'; c <= 'z'; c++)
        {
            tmpIndex[c] = N + 1;
        }

        for (int i = N - 1; i >= 0; i--)
        {
            for (char c = 'a'; c <= 'z'; c++)
            {
                nextIndex[i + 1, c - 'a'] = tmpIndex[c];
            }

            tmpIndex[A[i]] = i + 1;
        }

        for (char c = 'a'; c <= 'z'; c++)
        {
            nextIndex[0, c - 'a'] = tmpIndex[c];
        }

        int[] cost = new int[N + 2];
        for (int i = 0; i <= N + 1; i++)
        {
            cost[i] = int.MaxValue;
        }

        cost[N + 1] = 0;
        int t = 1;

        bool[] f = new bool[256];


        for (int i = N - 1; i >= 0; i--)
        {
            f[A[i]] = true;
            bool b = true;
            for (char c = 'a'; c <= 'z'; c++)
            {
                b &= f[c];
            }
            cost[i + 1] = t;
            if (b)
            {
                t++;
                f = new bool[256];
            }
        }

        cost[0] = t;
        int len = cost[0];
        int index = 0;
        char[] ans = new char[len];
        //Console.WriteLine();
        for (int i = 0; i < len; i++)
        {
            for (char c = 'a'; c <= 'z'; c++)
            {
                if (cost[index] - 1 == cost[nextIndex[index, c - 'a']])
                {
                    ans[i] = c;
                    index = nextIndex[index, c - 'a'];
                    break;
                }
            }

            // Console.WriteLine();
        }

        Console.WriteLine(new string(ans));
    }

    public static void Main(string[] args)
    {
        new Program().Solve();
    }
}

class Scanner
{
    public Scanner()
    {
        _pos = 0;
        _line = new string[0];
    }

    const char Separator = ' ';
    private int _pos;
    private string[] _line;

    #region スペース区切りで取得

    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = Console.ReadLine().Split(Separator);
            _pos = 0;
        }

        return _line[_pos++];
    }

    public int NextInt()
    {
        return int.Parse(Next());
    }

    public long NextLong()
    {
        return long.Parse(Next());
    }

    public double NextDouble()
    {
        return double.Parse(Next());
    }

    #endregion

    #region 型変換

    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }

    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }

    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }

    #endregion

    #region 配列取得

    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = Console.ReadLine().Split(Separator);

        _pos = _line.Length;
        return _line;
    }

    public int[] IntArray()
    {
        return ToIntArray(Array());
    }

    public long[] LongArray()
    {
        return ToLongArray(Array());
    }

    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }

    #endregion
}

Measured memory_bytes: 35264