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

public class Program
{
    public static void Main(string[] args)
    {
        string shortestString = FindShortestString();
        Console.WriteLine(shortestString.Length);
    }

    public static string FindShortestString()
    {
        HashSet<string> combinationsSet = GenerateCombinationsSet();
        string shortestString = string.Empty;

        foreach (string combination in combinationsSet)
        {
            if (!shortestString.Contains(combination))
            {
                shortestString += combination;
            }
        }

        return shortestString;
    }

    public static HashSet<string> GenerateCombinationsSet()
    {
        HashSet<string> combinationsSet = new HashSet<string>();

        for (int i = 0; i <= 9999; i++)
        {
            string combination = i.ToString("D4");
            combinationsSet.Add(combination);
        }

        return combinationsSet;
    }
}