It's so simple in C# ...
But what's wrong ?

using System;
using System.Text;
using System.IO;

    class Program
    {
        static void Main()
        {
            StringBuilder seq = new StringBuilder();
            for (ulong i = 1; i < 100000; ++i)
            {
                seq.Append(i);
            }
            string str = seq.ToString();
            string j = Console.ReadLine();
            Console.WriteLine(str.IndexOf(j) + 1);
        }
    }

try 999...999 (200 nines)