using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

/// <summary>
/// Provides binary search implementations.
/// </summary>
public static class BinarySearch
{
    /// <summary>
    /// Performs Meguru-style binary search.
    /// <seealso cref="https://twitter.com/meguru_comp/status/697008509376835584"/>
    /// </summary>
    public static int Meguru(int ok, int ng, Func<int, bool> isOk)
    {
        while (Math.Abs(ok - ng) > 1)
        {
            var mid = (ok + ng) / 2;
            if (isOk(mid))
            {
                ok = mid;
            }
            else
            {
                ng = mid;
            }
        }
        return ok;
    }

    /// <summary>
    /// Performs Meguru-style binary search.
    /// </summary>
    public static long Meguru(long ok, long ng, Func<long, bool> isOk)
    {
        while (Math.Abs(ok - ng) > 1)
        {
            var mid = (ok + ng) / 2;
            if (isOk(mid))
            {
                ok = mid;
            }
            else
            {
                ng = mid;
            }
        }
        return ok;
    }

    /// <summary>
    /// Performs binary search
    /// to find the lower bound index of the specified value in the range.
    /// </summary>
    public static int LowerBound<X>(this IReadOnlyList<X> @this, X value, int lb, int ub, IComparer<X> comparer)
    {
        if (lb > ub) throw new ArgumentOutOfRangeException(nameof(ub));

        while (lb != ub)
        {
            var m = lb + (ub - lb) / 2;
            if (comparer.Compare(@this[m], value) < 0)
            {
                lb = m + 1;
            }
            else
            {
                ub = m;
            }
        }

        return lb;
    }

    /// <summary>
    /// Performs binary search
    /// to find the lower bound index of the specified value.
    /// </summary>
    public static int LowerBound<X>(this IReadOnlyList<X> @this, X value, IComparer<X> comparer)
    {
        return LowerBound(@this, value, 0, @this.Count, comparer);
    }

    /// <summary>
    /// Performs binary search
    /// to find the lower bound index of the specified value in the range.
    /// </summary>
    public static int LowerBound<X>(this IReadOnlyList<X> @this, X value, int lb, int ub)
    {
        return LowerBound(@this, value, lb, ub, Comparer<X>.Default);
    }

    /// <summary>
    /// Performs binary search
    /// to find the lower bound index of the specified value.
    /// </summary>
    public static int LowerBound<X>(this IReadOnlyList<X> @this, X value)
    {
        return LowerBound(@this, value, 0, @this.Count, Comparer<X>.Default);
    }

    /// <summary>
    /// Performs binary search
    /// to find the upper bound index of the specified value in the range.
    /// </summary>
    public static int UpperBound<X>(this IReadOnlyList<X> @this, X value, int lb, int ub, IComparer<X> comparer)
    {
        if (lb > ub) throw new ArgumentOutOfRangeException(nameof(ub));

        while (lb != ub)
        {
            var m = lb + (ub - lb) / 2;
            if (comparer.Compare(@this[m], value) <= 0)
            {
                lb = m + 1;
            }
            else
            {
                ub = m;
            }
        }

        return lb;
    }

    /// <summary>
    /// Performs binary search
    /// to find the upper bound index of the specified value.
    /// </summary>
    public static int UpperBound<X>(this IReadOnlyList<X> @this, X value, IComparer<X> comparer)
    {
        return UpperBound(@this, value, 0, @this.Count, comparer);
    }

    /// <summary>
    /// Performs binary search
    /// to find the upper bound index of the specified value in the range.
    /// </summary>
    public static int UpperBound<X>(this IReadOnlyList<X> @this, X value, int lb, int ub)
    {
        return UpperBound(@this, value, lb, ub, Comparer<X>.Default);
    }

    /// <summary>
    /// Performs binary search
    /// to find the upper bound index of the specified value.
    /// </summary>
    public static int UpperBound<X>(this IReadOnlyList<X> @this, X value)
    {
        return UpperBound(@this, value, 0, @this.Count, Comparer<X>.Default);
    }
}

public sealed partial class Program
{
    string source;

    List<int>[] indexesFromChar;
    int[] ranks;
    int n;

    void Prepare()
    {
        indexesFromChar = 26.MakeArray(_ => new List<int>());

        var rankFromChar = new int[26];
        ranks = new int[source.Length + 1];
        var rank = 0;
        var count = 0;

        for (var i = source.Length - 1; i >= 0; i--)
        {
            var c = source[i];

            ranks[i] = rank;
            indexesFromChar[c - 'a'].Add(i);

            if (rankFromChar[c - 'a'] == rank)
            {
                rankFromChar[c - 'a']++;
                count++;

                if (count == 26)
                {
                    rank++;
                    count = 0;
                }
            }
        }

        foreach (var indexes in indexesFromChar)
        {
            indexes.Reverse();
        }

        n = rank + 1;
    }

    char[] SolveCore()
    {
        var target = new char[n];
        var si = 0;

        for (var ti = 0; ti < n; ti++)
        {
            var c = 'a';

            for (; c <= 'z'; c++)
            {
                if (si >= source.Length) break;

                var indexes = indexesFromChar[c - 'a'];
                var ii = indexes.LowerBound(si);
                if (ii == indexes.Count)
                {
                    si = source.Length;
                    break;
                }
                else
                {
                    var sj = indexes[ii];
                    var restCount = n - ti - 1;
                    if (ranks[sj] + 1 == restCount)
                    {
                        si = sj + 1;
                        break;
                    }
                }
            }

            target[ti] = c;
        }

        return target;
    }

    string Solve()
    {
        Prepare();
        return new string(SolveCore());
    }

    void Read()
    {
        var a = scanner;
        source = input.ReadLine();
    }

    public void EntryPoint()
    {
        Read();
        WriteLine(Solve());
    }
}

#region Custom Library
public static class TemplateExtension
{
    public static X[] MakeArray<X>(this int count, Func<int, X> func)
    {
        var xs = new X[count];
        for (var i = 0; i < count; i++)
        {
            xs[i] = func(i);
        }
        return xs;
    }

    public static int[] Range(this int count, int start = 0)
    {
        return count.MakeArray(i => i + start);
    }

    public static string Intercalate<X>(this IEnumerable<X> @this, string separator)
    {
        return string.Join(separator, @this);
    }

    public static void ForEach<X>(this IEnumerable<X> @this, Action<X, int> action)
    {
        var list = @this as IReadOnlyList<X>;
        if (list != null)
        {
            var count = list.Count;
            for (var i = 0; i < count; i++)
            {
                action(list[i], i);
            }
        }
        else
        {
            var i = 0;
            foreach (var x in @this)
            {
                action(x, i);
                i++;
            }
        }
    }
}

public sealed class Scanner
{
    readonly TextReader reader;

    /// <summary>
    /// Reads next word separated by spaces.
    /// </summary>
    public string Word()
    {
        var sb = default(StringBuilder);
        var firstChar = default(char);
        var count = 0;

        while (true)
        {
            var r = reader.Read();

            if (r == '\r')
            {
                if (reader.Peek() == '\n') reader.Read();
                break;
            }
            else if (r == -1 || r == ' ' || r == '\n')
            {
                break;
            }
            else
            {
                var c = (char)r;

                switch (count)
                {
                    case 0:
                        firstChar = c;
                        count = 1;
                        break;
                    case 1:
                        sb = new StringBuilder();
                        sb.Append(firstChar).Append(c);
                        count = 2;
                        break;
                    default:
                        sb.Append(c);
                        break;
                }
            }
        }

        switch (count)
        {
            case 0:
                return "";
            case 1:
                return firstChar.ToString();
            default:
                return sb.ToString();
        }
    }

    /// <summary>
    /// Reads next word as <see cref="int"/>.
    /// </summary>
    public int N()
    {
        return int.Parse(Word());
    }

    /// <summary>
    /// Reads next word as <see cref="long"/>.
    /// </summary>
    public long L()
    {
        return long.Parse(Word());
    }

    /// <summary>
    /// Reads next word as <see cref="double"/>.
    /// </summary>
    public double F()
    {
        return double.Parse(Word());
    }

    public int[] Ns(int count)
    {
        return count.MakeArray(_ => N());
    }

    public long[] Ls(int count)
    {
        return count.MakeArray(_ => L());
    }

    public double[] Fs(int count)
    {
        return count.MakeArray(_ => F());
    }

    /// <summary>
    /// Reads next line and splits it by spaces.
    /// </summary>
    public X[] Words<X>(Func<string, X> func)
    {
        return reader.ReadLine().Split(' ').Select(func).ToArray();
    }

    public Scanner(TextReader reader)
    {
        this.reader = reader;
    }
}

public partial class Program
{
    readonly TextReader input;
    readonly TextWriter output;
    readonly Scanner scanner;

    void WriteLine(int value)
    {
        output.WriteLine(value);
    }

    void WriteLine(long value)
    {
        output.WriteLine(value);
    }

    void WriteLine(double value)
    {
        output.WriteLine(value);
    }

    void WriteLine(char value)
    {
        output.WriteLine(value);
    }

    void WriteLine(string value)
    {
        output.WriteLine(value);
    }

    public Program(TextReader input, TextWriter output)
    {
        this.input = input;
        this.output = output;
        scanner = new Scanner(input);
    }

    public static void Main(string[] args)
    {
#if DEBUG
        using (var writer = new VainZero.IO.DebugTextWriter(Console.Out))
#else
        var writer = Console.Out;
#endif
        {
            new Program(Console.In, writer).EntryPoint();
        }
    }
}
#endregion


Measured memory_bytes: 16608